My iPhone SE magnifier is stuck in selfie mode.

If you open Magnifier, you should see a control on the left side that looks like a camera. Tapping on that allows you to choose which camera you want to use. If that is not working, then it is possible something went wrong when the camera was replaced. Did Apple or an Apple Authorized service center make this replacement? If so, contact them about checking the repair.

On my iPhone 16 Pro Max with iOS 18.4.1, this is what I see when I open Magnifier. The camera button I was referring to is above the icon that is above the wording Activities. If you are not seeing it, then tap on the Settings icon, then in the list under Secondary Controls, see if the Cameras is listed. If not, then look under Other controls and see if it is listed there with a + in front of it. If so, then click on the + and it will get added to the Secondary controls list and should appear on the screen after tapping on Done in Settings. If you cannot see that, then I suggest returning to Best Buy.

Good luck.