Can't update

Janaque​  If you want to figure out what's taking up so much space, open a File Explorer window, click This PC in the left panel, double-click the C drive, and start checking the sizes of the folders inside.  For any folder that's larger than you expect, open it and check its subfolders until you find the problem.  Feel free to list your findings here.

I can help you direct the Sims 4 user folder to D, if you want.  That won't free up much space on C, but it would allow you to use the mods and custom content you've saved.

You could also uninstall the game on C and reinstall to D.  If you're going to go that route though, I'd suggest doing a clean uninstall with Revo Uninstaller (the free version is fine) to catch any stray data that's taking up space on C.  How to do so is covered in the accepted solution of this thread:

[CURRENT ISSUE] Sims 4 DLC shows as installed, won't load in-game | EA Forums - 11831673

I'm not saying you need to do this, at least not now, only that it's an option.  I'd still suggest hunting for the extra data on C first.