The tail of the log for the database “DBName” has not been backed up

The error message you are getting tells you exactly what you need to do if you don’t care about the existing database or log.

RESTORE DATABASE DAtabaseName FROM DISK = 'C:\DBName-Full Database Backup' 
WITH REPLACE

In SQL Server Management Studio (Tasks > Restore), you can add the WITH REPLACE option by opening the page “Options” on the left side and ticking “Overwrite the existing database”.