iphone back up

See this link --> How to back up your iPhone, iPad, and iPod touch - Apple Support

You should actually be backing up more often (light nightly) as opposed to just before you get a new phone.

Go to Settings > Your Name, iCloud, etc, Tap iCloud and turn on iCloud Backup. In doing so, you're phone will backup every night when it is plugged in to charge and connected to Wifi.