This mod allows you to reach level 20, but you must multiclass so that no single class exceeds 12. 
While this will not get you higher level spells and abilities (because there are no such spells and abilities in game currently, they have to be modded in), it still allows a wide variety of powerful multiclassed builds.
(Notes:
- Take the 12th level last, because otherwise the game will try to level your class to 13 on next level-up and crash. So like this: Fighter 11/Barbarian 8/Fighter 12;
- Multiclass spell slot table does not go beyond 12, so even if you take 20 levels in casting classes, you will still get no more than 12 levels worth of spell slots;
- Same rules apply to companions).

The mod comes in three variants:

Option 1: Regular leveling
XP required for each level are the same as vanilla for 1-12, and build off vanilla progression for 13-20. However, there is of course not so much XP available in the game itself, so this option is not for those who  want to get to 20, but those who want to play "fairly" and see if they can squeeze 1 or 2 more "honest" levels out of vanilla BG3, by doing all there is to do.

Option 2: Faster leveling
XP required for each level is halved, and higher level XP is more lenient. You should be able to reach at least level 18, maybe more depending on how much XP is in the game.

Option 3: Instant leveling
To reach level 20, only 200 XP are needed. This option is mainly for testing out level 20 builds, or breezing through the game on subsequent playthroughs. Not recommended for serious gameplay.

INSTALLATION:


1. Download chosen file MANUALLY, DO NOT USE ANY MANAGERS/DOWNLOADERS.
2. Put the file in the game's INSTALLATION FOLDER (e.g. for users with Steam, it will be something like steamapps/common/Baldur's Gate 3)
3. Right-click on file
4. Choose Extract here
The file should now unpack and AUTOMATICALLY create all the necessary folder structure.
It should look like this:
[Baldur's Gate 3]/Data/Public/Shared/Stats/Generated/Data/XPData.txt
[Baldur's Gate 3]/Data/Public/SharedDev/Stats/Generated/Data/XPData.txt

ATTENTION: This is the step many people have trouble with. You should check that the filepath is correct. If it is not correct, for some reason (you extracted into a different folder, or in some older versions of the file an extra folder with the mod name may be created on top), then adjust the folder structure manually.

6. Now launch the game, disregarding the "warning message" in the Larian launcher.

The changes should now be in effect. 

Compatible with my "Fast XP to level 6" mod (install the level 6 option on top of this mod, overwrite when prompted).

KNOWN ISSUE (Fixed):

Once you level up past level 12, this may cause an issue with vendors' inventories, since vendor tables seem to be tied to character level (for some stupid reason), and in such a way that levels higher than 12 are not always accommodated. A fix is included by default in mod versions 1.3 and higher, and is also available separately under "Optional Files"(you may need to take a long rest to refresh vendor inventories).