I’m just guessing, but I think you download the new moc binaries and then try
export DFX_MOC_PATH=<path to new moc>
and then use dfx as before.
Update:
I just tried (in a new dfx project)
export DFX_MOC_PATH=/home/crusso/motoko/bin/moc
dfx deploy
‘/home/crusso/motoko/bin/moc’ is where my locally built moc lives and it worked fine.
You might have to make the moc you download executable first.
(Related post: DFX deploy with custom Motoko version)