The ContinuaCI installer supports upgrading in place, there is no need to uninstall before installing a newer build. The installer will detect an existing installation and ask if you want to use the existing settings or provide new settings. In some instances you may need to upgrade the Server and the Agents at the same time, check the release notes for each new build to confirm whether this is necessary.
Downgrading
ContinuaCI does not support downgrading. Whilst it is technically possible, database schema changes could result in data loss. Backup your database before installing an older build.