If not, login to Spotify Desktop and unlink Facebook from within the app (Preferences > Disconnect from Facebook).
After that you'll be able to reconnect it, and you'll need to use your spotify username/password to login.
If you created your Spotify account with Facebook, you need to create a new account with a different email address from the one you use for Facebook. That will give you a Spotify username, so you won't have to rely on a Facebook account.
Once you have this new account, fill out the contact form, and someone from Spotify will be happy to help you migrate your Premium privileges (if you have them) to this new account.
If you like you can link this new account to your Facebook after the switch.