About iOS version in iPhone 6 to update 13

Either return the Apple Watch

https://www.apple.com/shop/help/returns_refund

Or get a newer iPhone. You need a minimum iPhone 6s, or iPhone SE (1 or 2), or iPhone 7, 8, X, XS, XR, 11 or 12

if it's running watchOS6 or watchOS7 then you can't by any means, it require ios13 and iphone6 does not support that.

As already stated you need at least ios13 and you phone won't go any higher than ios12.