How can you restore a previous back up on your iPhone?

You can only restore a previous backup if you have archived it in iTunes or if it is one of the two backups that iCloud stores for each iOS device that you have. If you want to restore an iCloud backup, you have to erase the device first. When you restore a backup, you overwrite the current contents on your device.

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