Backing up my iphone

I have booked an appointment with yourselves next week and it said in the email to ensure that my phone was backed up. The issue I had was that my phone is unresponsive to touch screen, so I removed my sim from that phone and am currently using my old iPhone 6. I now have half my information and pictures etc on this iPhone 6 but also half on my old iPhone XS max. How do I back up both phones to ensure nothing gets lost?