Getting your 1SE to a new device is super easy! The first step is to make sure you have an iCloud backup with 1SE on it. The best bet is to do one last backup on your old device. Sadly, there's not a way to view what's inside an iCloud backup once it's done. So best to be safe and do one last backup!
VERY IMPORTANT: Please keep in mind logging into iCloud isn't the same as an iCloud backup. 1SE is NOT saved in your iCloud account. Only basic data like notes, mail, and contacts are. You'll need to do a backup via iCloud or iTunes.
Via: Apple Support
Once you have your iCloud backup, simply follow these steps if on a new device:
Plug in your phone, connect to your Wi-Fi, and grab a cup of coffee. Restoring will take some time.
If you're restoring on a new device, follow the onscreen setup steps until you reach the "Apps & Data" screen, then tap "Restore from iCloud Backup".
Sign in to iCloud with your Apple ID.
Agree to the Terms and Conditions.
Choose the most recent backup that has 1SE in it.
Relax and let the restore finish. It'll show a time estimate in a bit.
If you're restoring on a current device, follow these steps:
Plug in your phone, connect to your Wi-Fi, and grab a cup of coffee. Restoring will take some time.
Go to Settings > General > Reset, then tap “Erase all content and settings.”
On the "Apps & Data" screen, tap "Restore from iCloud Backup", then sign in to iCloud. Accept the Terms and Conditions.
Proceed to “Choose Backup”, then choose from a list of available backups in iCloud.
Choose the most recent backup that has 1SE in it.
Relax and let the restore finish. It'll show a time estimate in a bit.
Want a little more detail or information? Here's a super detailed help article from Apple on restoring.
Visuals always help! Here's a step by step visual guide for iCloud backups:
Using iTunes to restore instead? Here's how you restore using iTunes.