I got an apple watch series 5 from my neighbor

Hi Brolo14,

We'll be glad to help out. If the watch hasn't been unpaired and erased, you'll need to follow the steps outlined here to complete that: Unpair and erase your Apple Watch - Apple Support

"How to erase your Apple Watch if you don't have your iPhone

1. On your Apple Watch, tap Settings > General > Reset > Erase All Content and Settings. 
2. 
3. 
   
4. Type your password if prompted.
5. For GPS + Cellular models, choose to keep or remove your cellular plan.

• If you want to pair your Apple Watch and iPhone again, keep your plan. 
• If you don't want to pair your Apple Watch and iPhone again, remove your plan. If you aren't going to pair with a different watch or iPhone, you might need to contact your carrier to cancel your cellular subscription. 

1. Tap Erase All to confirm. This resets your Apple Watch to the factory settings.

Now you can set up your Apple Watch again. You'll need the Apple ID and password associated with the watch turn off Activation Lock.

Erasing all content and settings erases all media, data, and settings from your Apple Watch, but it does not remove Activation Lock. To remove Activation Lock, unpair the watch first. Before your watch is erased, a backup of its contents is created on your iPhone."

We hope this helps you out.

Cheers.