Q: I bought a 5S from eBay. I'm trying to restore an icloud backup, but phone has an older iOS than that which created the backups. How can I activate the phone?

Hello daveyslocker,

Thanks for using Apple Support Communities.

If you want to restore from a backup but are unable to because the iPhone has an older iOS installed, then you will need to first update iOS by using iTunes.  Once updated you can then restore from a backup

Update your device using iTunes

If you can’t update wirelessly, you can update with iTunes. If your computer uses the Personal Hotspot on the iOS device that you’re updating, connect your computer to a different network.

1.  Install the latest version of iTunes on your computer.
2. Connect your device to your computer.
3. Open iTunes and select your device.
4. Click Summary, then click Check for Update. 
   
5. Click Download and Update.

Update the iOS software on your iPhone, iPad, or iPod touch - Apple Support

iCloud: Restore or set up your iOS device from iCloud

Take care.