Factory reset watch series 5

Hi guys ,

I have a Apple Watch Series 5.

yesterdat I bought a series 7.

I start the new watch and I transfer all data to the new one.

when was finish I put the new one in my wrist and then I did a big mistake…

so I went in my iPhone 12 in the watch app and I erase the series 5 without unpair. So basically I erased the old watch and I forgot to unpair the old one.

now my new is working perfectly but my old one is still paired but no more in the iPhone. Every time I open my old one, and look in settings my name is in there and I can’t reset him on factory settings

how can I reset him because I selling him and I don’t won’t my data in the old watch. Please I need to reset as a new watch

thanks a lot

Thiago Ardizzoia from Ireland