How can I get my back up data if I erase my iPhone

iPhones — by default — backup automatically to iCloud, so unless you explicitly disabled this feature, you’re in all liklihood in fairly good shape.

Don’t let the term “Restore from Backup” scare you.

Apple recognizes that mobile devices will unfortunately be lost, damaged, or stolen with some regularity; so the restoration processes are designed to be simple and easy-to-use by most individuals, AND they work VERY well.

Restore your iPhone, iPad, or iPod touch from a backup - Apple Support

only way is if you took a backup on a computer or to icloud

also if you set it to sync with icloud then the synced data such as contacts and the likes is still there and will come back when the phone is set to use the same apple account, and set to use icloud