If you read the description of the mod, it states the following:
4. Derivative Works / Modification
This Mod may not be decompiled, reproduced and/or modified under any circumstances without prior written permission of the Owner. This Mod is not open source.
This means no one can update the mod for you unless they have permission from the creator himself. Yet here lies another issue, namely that the owner has seemingly disappeared off the face of the earth a few years ago (or at least removed his existence entirely from all things Minecraft). This means there is no permission to be given, and thus Witchery must be left where it stands right now.
People have made alternatives to Witchery (maybe not for 1.12.2, but I do remember some solid efforts being done for 1.10 at some point), but none of the assets or code provided by Witchery is able to be used for them.