Always backup in case. You will need to put iPhone into recovery mode and update which should put back stable release iOS 14.6 and then install 14.7 using beta profile. Or get the IPSW for 14.7 b3 and use recovery mode to update straight to it. Just don't confuse DFU and recovery mode. DFU mode does not have a "Update" option if you want to preserve your data.
It might be easier to restore if you want to avoid issues after downgrading.