Let me guess, you don't use the steam workshop ?
Because the workshop auto. update all mods so long you are online.
But that is curious, when you don't know how to update, how did you install manual the mods ?
It is basicly the same, just delete the old mod folder, and install the newly downloaded one.
Normaly inside the zip-file is the mod folder, just copy/unzip into the Mods folder at your rimworld installation. Better delete the old mod folder first, so no old files are left when the mod author change the file structure.
I suggest when you don't use the steam version to read this topic
How to download workshop mod's as non-steam user.
https://ludeon.com/forums/index.php?topic=52498.0RimPy together with SteamCMD is a very comfortable mod manager with workshop download/update feature.