I need help freeing up storage in order to complete a software update

Do you have a Mac or a Windows machine you can connect your phone to?

If neither, you can get an OTG OnTheGo thumbdrive to get your photos and videos copied to. Recommend you make two separate copies to the thumbdrives.

If a Mac, Image Capture lets you manage getting the photos and videos onto your Mac.

Music transferred from another computer does not transfer well to a secondary computer through the iPhone.

https://support.apple.com/guide/iphone/transfer-files-iphone-computer-iphf2d851b9/ios

Covers Windows in more detail.

Does this pertain to an Apple Computer or your iPhone 6S, Plus as per your profile??