Title

A solution I have found is If a mod only has base game dependancy this seems to work:
1. Download the 2.0 version of the mod
2. Extract the zip file
3. Go into the mod folder after extracting and edit the info.json file.
4. edit the version from 2.0 to 2.1
5. Zip the folder after the editing is done and move it to your factorio mods folder replacing or overwriting the previous one there.

Note: On windows, the factorio mods folder usually is %APPDATA%\Factorio\mods which you can enter after hitting Windows + R

It depends on what specific parts of the base game a mod depends on.
This mod specifically is fine for that, but mods that add recipes are likely broken.

Would be nice to have an official 2.1 though. Guess i'm just updating it for myself for now.

FYI, this is not a 1:1 update to 2.1.
There are a couple of lines that need to be fixed for it to work properly.

belt.neighbours needs to be replaced with belt.underground_belt_neighbour

Pls mr mod maker I need this

https://mods.factorio.com/mod/belt-reverser-forked
This fork also fixes a couple issues as well