Back up

Don't worry you're only deleting the back-up and not the original data, I always keep my most recent back-up only to save space

You might also want to back-up your phone via iTunes or iCloud (the other backup method to the one you are using)

You can find out the steps at How to back up your iPhone, iPad and iPod touch – Apple Support

In addition to what was said above, see also - Manage your iCloud storage - Apple Support.