how do i sync my siri remote?

What do you want Siri remote to sync?

Read here: Use your Siri Remote or Apple TV Remote with Apple TV (4th generation) - Apple Support

You likely mean "pair" not "sync"?

The Siri Remote comes paired to your Apple TV 4 out of the box. If it doesn't seem to work, try charging it for about an hour. Otherwise, check your Settings on ATV.

Apple TV User Guide - website https://help.apple.com/appletv/#/

Apple TV User Guide - iBook https://itunes.apple.com/us/book/apple-tv-user-guide/id1050195582?mt=11

1. Have you tried charging the remote.
2. Have you tried pressing the menu and up button for a few seconds while holding it an inch or two away from the Apple TV, if successful you should see an onscreen message saying the remote has been paired.
3. If you have an older remote from an earlier model Apple TV or MacBook, you may be able to use it to restore the Apple TV which in many cases has been enough to re-enable the Siri remote. You will need to complete set up to undertake a restore.
4. If you have a USB-C cable you can try restoring it by connecting it to iTunes on a computer.
5. If you have tried all these steps, don't have an earlier remote or a USB-C cable or using them is too inconvenient, return it to the store from which you purchased it for a replacement or to have them pair it for you.