I’ve already set up my new phone but I did a recent backup on my old phone how do I transfer this new back up from my old phone to my new phone that’s already set up?

The only way to restore a backup to your already set up phone is to completely erase it and start again from the Hello screen, but this time transfer your data and apps from the chosen backup .

This is standard procedure if the new phone needs an update before you can transfer, eg the new phone is on ios16 from the store but the old phone is on ios17.

You will of course lose any new data you accumulated on the new phone since the backup

p.s. that is settings, general, transfer or reset iphone, erase all content and settings.