You can do this. It appears as a separate symbol in your castable spells and you just apply it again. As long as you maintain concentration it'll last until you next take a long rest (at least as of the current version).

If the hexed creature makes a save for that ability he rolls 2 dice and picks the worse if nobody forcea a check(web for eex, laughter for wisdom etc) it does nothing

It's a useless waste of time for 99% of gameplay and should be streamlined to affect ALL abilities automatically either as full disadvantage or a smaller flat penalty. It is potentially useful cursing highest enemy ability of DEX or STR before using SHOVE. Other uses are so questionably and marginally useful, that they don't even warrant mentioning.

Kinda late to the party so I'm not sure if you'll see this, but both of these replies are wrong and seem to have misunderstood the spell, which I don't blame them for because it is a very complicated spell.

Hex's debuffs only apply to ABILITY CHECKS, not saving throws or attack rolls or anything else. Hex will not generally help you very much in combat with these debuffs because there aren't a lot of skill checks in combat, there are mostly ability checks and attack rolls. The most you'll get out of it is maybe stopping an enemy from using shove if you hex their strength, but really it's more for the extra d6 of damage if you're looking at it from a purely combat perspective and whatever stat you hex really does not matter.
I will however point out that initiative, the roll you make upon entering combat to determine turn order for every character entering combat, is in fact a Dex ability check. That means you can sneak up and hex dex somebody to enter combat, and they will roll their initiative with disadvantage which gives them a much larger chance of being put at the end of the turn order. This can be very powerful against stronger low dex targets as an opening move, since your entire team will be more likely to go before they will and can coordinate their attack much easier, and you might even get a surprise round on top of it which means your entire team really gets to bushwhack the target good before they can even do anything. This is probably its most powerful potential combat use aside from the extra d6.

Hex debuffs probably wont get to see its full potential in BG3 because of it being a fixed story with limitations from the game engine, but Hex in actual DnD can do SO many things outside of combat with these debuffs. You can just hex your Barbarian if you want to be an *******, and now every time he wants to make a strength check, say to break down a door or lift a heavy object, he has to do so at disadvantage. This can serve as a very powerful tool in a lot of story elements too, since it can potentially let you steer the coarse of the story by subtly hexing the right targets at the right time to make them mess up important actions, but we'll have to see if those situations appear in BG3 or not.

Also this game some terminology that might take a while to grasp. Advantage/Disadvantage means you roll twice and take the higher/lower roll respectively. This effect does not stack, and if any instance of advantage and disadvantage are applied at the same time you simply roll normally. This DOES NOT cancel out for imbalance either, which is a mistake people often make, so 5 advantages and 1 disadvantage still rolls normally and not at advantage.
Ability checks are when you have a check against the stat itself, being it STR, DEX, CON, INT, WIS, or ,CHA. These checks do not use a skill, such as perception, investigation, arcana, or persuasion. Those are more like subcategories of abilities and will have their own rolls defined more specifically, such as an "Arcana check" or "Persuasion check".
A good example of the differences would be lifting the heavy boulder covering the Harpers chest near the start of the game, which is a STR check, whereas an action like Shove during combat is specifically an Athletics check. Both rely on STR, but they are different rolls and can have different values due to modifiers.

Sacred Flame causes enemies to make a save. That is why Hex does not effect it.

Hex debuffs ability CHECKS, not saving throws. Saving throws are a different thing.

So it wont make it easier to hit with fireball, or hold person. But if you debuff strength, it'll be harder for the target to use Shove, for example.

Basically useless, yet slowing down the casting so much...

some of laezhel's attacks have con saving throw

hex isn't useless, it adds 1d6 dmg to warlock's attacks

DEX now is firaball save, so there's that. I'm doing a playthrough now with a bard and a great old one warlock and they hve a lot of WIS save spells, so hex will mostly be used for that. You just gotta check what resists what and think ahead.

In reply to the person who posted on the 6th of Jan 2023:

Hex only debuffs ability CHECKS, not saving throws. So it wont make it easier to hit fireball or hold person.

This is disliked for no reason. A great suggestion