cd portal/
git submodule update --init # if necessary
cd submodules/motoko
git checkout 0.7.6 #or whatever new tag you desire
cd ../..
git add submodules/motoko # add the change to the submodule
Now replace static/moc-interpreter-0.13.6.js with static/moc_interpreter-0.13.7.js, downloaded from Motoko release page. It would be better if this wasn't checked in, but, for now, it is.
Edit static/load_moc.ts to use the correct version of the interpreter and base libs.
Finally, edit docs/motoko/version.md to report the current version of Motoko.
git add -u
git commit -m "chore: updating motoko doc"
git push