The form shows Protocol, Encryption, Server, Port, User, Password and Remote FTP folder, and a password SyncNow already keeps shows as Stored securely: click Replace to type a new one. Test connection tries the details and says Connected with the time it took, or why it failed.
Save hosting connection tests the FTP connection with the new details before saving: if the server refuses them, nothing is saved and SyncNow says why. If the test gets no answer within 20 seconds, the connection is saved anyway and a message at the bottom of the window says, for a few seconds, that it was not verified.
If you change the server or the user, type the password again, because the stored password belongs to the old server and user. If you keep the stored password and the same server and user, there is nothing to test on your PC: the server uses the stored password at the next operation.