If you haven't already upgraded to the 4.19 v2, you'll follow the same directions as in the release thread, as you'll still need to create the f2fs partition for the storage.
If you have already upgraded to the 4.19 v2 that was released a few days ago, you can just run the sysupgrade-itusbridge.tar.gz or sysupgrade-itusrouter.tar.gz directly from luCi or sysupgrade CLI. Please do NOT save the existing configuration!
You'll need to check/change the distfeeds.conf as well, since I'm still working out the process for these things and didn't bake it in first (it'll be fixed in future revisions):