Yes it is always a great idea to do a backup.
You should be able to just run the installer and do a regular install and it will upgrade the service and the website files without damaging your setup. But just in case you should run a backup first.
You only need a custom install if you manually installed dbFront initially.
The folders that should be backed up are:
- Service Folder: c:\program files(x86)\dbfront
- Website Folder: c:\inetpub\dbfront
- Data / Settings: c:\data\dbFront