Q: how do i change the apple id on my iPhone

For what exactly?  There are 4 places an Apple ID is used in an iPhone, and there's no requirement they be signed into the same Apple ID for all of them.

To change it Sign out of it in Settings for whatever it is you want to change, and Sign in with the one you want.

Settings->iCloud

Settings->iTunes & App Stores

Settings->Messages

Settings->Facetime.

For iCloud scroll all the way to the bottom of the screen, and there should be a red "Sign Out" option.

For the other 3, just Tap on the Apple Id on each and then on sign out.

For more specific instructions:

Sign in with a different Apple ID on your iPhone, iPad, or iPod touch - Apple Support