Hi SiobhanNJ,
This article will help you set up replacement AirPods: Set up your replacement AirPods or charging case.
Set up your AirPods or charging case
1. On your iPhone, iPad, or iPod touch, go to Settings > Bluetooth and tap the Information button
next to your AirPods.
2. Tap Forget This Device.
3. Tap Forget Device, and tap again to confirm.
4. Put both AirPods in your charging case.
5. Open the lid and check the status light. It should flash amber.
6. Press and hold the setup button on the back of the case for a few seconds. The status light should flash white.*
7. Go to the Home Screen on your iPhone, iPad, or iPod touch.
8. Open the case—with your AirPods inside—and hold it next to your iPhone, iPad, or iPod touch. A setup animation appears on your screen.
9. Tap Connect, then tap Done.
If you need more help, reply with some additional information on what happens when you try to set up your AirPods.
Regards.