Creating space on full WD backup drive

I’m a complete beginner to the world of backups, so please be gentle!

I recently bought a WD Passport external drive to attach to my Windows PC with the idea of leaving it plugged in and backing up once a day, using the WD Backup software provided. Foolishly I set it to backup everything on the PC, not realising that the drive itself only had slightly more capacity than the amount that was to be backed up.

Now of course I am rapidly running out of space on the drive, and would like to remove some unnecessary folders on the backup drive to free up some space. How am I able to do this? I’ve resisted the urge to delete the folders in Windows Explorer in case this causes problems with the rest of the data.

Any suggestions gratefully received!

I use Windows File History with an ancient 1.5TB WD15EADS hard disk and it handles it fine