In /opt/discord-canary (remove -canary for stable version or replace with -ptb for test release) edit the file resources/build_info.json that should look like:

{
  "releaseChannel": "canary", 
  "version": "0.0.xxx"
}

You can "upgrade" the version by replacing the xxx above with the latest version, and trick the launcher. All is reset in a future update.

ref: Discord_asks_for_an_update_not_yet_available_in_the_repository