Given a deb installed MAAS at a remote server, say maas-testing.internal, if you can ssh root@maas-testing.internal then you can use the remote-reinstall script in MAAS’ utilities directory.
For purposes of testing UI changes, then you will first need to build the UI:
cd src/maasui
git checkout <whatever-hash>
cd ../..
make clean-ui-build ui
The actual reinstallation script is very straightforward:
utilities/remote-reinstall maas-testing.internal
This will transfer files in your current workspace via ssh root@maas-testing.internal, and restart MAAS.