I want to remove the CMS of my silverstripe web system and host it separately in a different server. What is the best way to do this?
As with all web application, back up the source code, assets and database... then restore them on the target server. There is a utility to aid you with this called sspak. I think you still need to handle .env files manually as these contain the most sensitive information (i.e. database passwords).
Other considerations might be if you have crons installed to ensure they are migrated. Also be sure to watch for anything whitelisted/authorised by the existing servers IP.