Hi
Follow the instructions here to switch your Apple Watch between your old and new iPhone:
Switch your Apple Watch to a new iPhone - Apple Support
If you continue to use your Apple Watch after you no longer have access to your iPhone 6s, then it will not be possible to retain any data that is accumulated on your watch before you pair it to your new iPhone.
It is also not possible to sync it with an iPad or any other device in the interim period or to export the data in any other way.
If you do not unpair your Apple Watch before stopping using your old iPhone, then, in order to pair your Apple Watch with your new iPhone, you will first need to manually erase it by following these steps:
On your Apple Watch, go to: Home screen (via a single press on the Digital Crown) > Settings (cog icon) > General > Reset > Erase all Content and Settings.
Then pair it to your new iPhone via the standard process, choosing to Restore from Backup, if offered the option:
Set up your Apple Watch - Apple Support