Apple watch still paired to an iPhone I no longer have

I used to have an iPhone from work and my Apple watch was paired to it. Now that I've bought a new iPhone and given back my old iPhone I am unable to pair my apple watch to the new phone. My Apple watch says: "wrong code, go to Apple Watch app and..." There is no chance to enter the code manually. But in the App I can only start to pair a new device, but the watch is still paired. What steps do I have to do? My Apple Watch is from series one and I don't know the software version nor how to determine it.