New Bluetooth headphone sync

i cannot remove the old headphones

What do you mean - it won't let you delete the old headphones or you can't find out how to do it?

On my watch it's Settings -> Sensors and accessories -> Select the headphones you want to remove -> Remove
(my translations, actual wording might be slightly different)

Also, I think you are able to have more than one set of headphones paired at the same time so you shouldn't need to remove the old ones in order to pair the new ones. (Settings -> Sensors and accessories -> Add new)

Thank you, found it