Haven't updated a mod on the Workshop for a while, but from what i recall..
- Need to be not (!) subscribed to your mod.
- Have your (unpacked) mod folder and *.mod file inside the "mod" folder in Documents.
- The *.mod file should include the "path" instead of "archive" line and no "*.zip" for the mod name.
- Do not have any older version of the mod in the folder (file, folder or zip).
The mod has to have the same file name as the old version, if i remember correctly.
- Don't have a descriptor file included.