Hi,
If you mean 4.5 or 4.4? I highly doubt you'll be able to. Not sure the reason, but I'd avoid doing so.
After completed install you need to restart the server.
I believe in /usr/src/fusionpbx-install.sh/debian/resources/config.sh you could change system_branch but ...
If you have problems with FS you do need to create PAT through Signalwire then add the token in config.sh in switch_token field.
After you edit /switch/repo.sh and add the username there.
If you still encounter problems, do chmod +x repo.sh then do ./repo.sh and reboot.