How to definitively clear away database before restoring from new version of backup file
Just use WITH REPLACE in your restore command and it will overwrite the existing databases on the destination.   Simple, and done.   You can DROP first, but its not necessary.  

WITH REPLACE is the T-SQL keyword that corresponds to "overwrite the existing database" in the restore database GUI (added for future readers of this thread)

Oh...and are you running an RC version of 2017?  RTM isn't until next Monday 10/2...just an afterthought.