I have Manjaro Budgie on one SDD.
Can I replace Budgie desktop environment to Cinnamon desktop environment ?
of course you can.
do you not know how
I know how with Ubuntu …but ,.,not with Manjaro.
Arch Linux is totally differnet.
Install all Budgie related packages, then uninstall all Cinnamon related packages.
Or simply backup your data and reinstall Budgie from scratch.
Have you done a search of the Manjaro wiki under desktop environments?