Iphone 6s stuck on recovery mode

Connect to a PC, with iTunes or a Mac, and first select Update. If that does not work then you will have to use Restore.

Restore deletes EVERYTHING on the device and restores it to the factory settings.

If you have an iCloud Backup, then you can restore from it:

https://support.apple.com/en-us/HT204184#icloud

Who replaced the battery?

If it was done by someone other than Apple or an authorized service provider, and restoring iOS doesn't fix it, you're on your own. Talk to whoever did the work.