Install the Updater with Prism Launcher or MultiMC

Download the GR Updater Mod from https://files.thegameroom.xyz/minecraft/modded-season-2/gr-updater-mod-1.0.0.jar

From the the launcher, select the Modpack and press the "Edit" button.

This will open another window - in that window, tap "Mods" on the left-hand-side, then tap "Add file" on the right-hand-side.

You can then select the gr-updater-mod-1.0.0.jar file you downloaded at the beginning.

You can now launch the Modpack, and you should be prompted with a screen to install the latest updates to the modpack.