About iOS version in iPhone 6 to update 13
How can I connect Apple Watch series 3 in iPhone6
How can I connect Apple Watch series 3 in iPhone6
if it's running watchOS6 or watchOS7 then you can't by any means, it require ios13 and iphone6 does not support that.
if it's running watchOS6 or watchOS7 then you can't by any means, it require ios13 and iphone6 does not support that.
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
what should I have to do to connect watch series 3 on iPhone 6 and my iOS is up to date IOS 12.5
As already stated you need at least ios13 and you phone won't go any higher than ios12.
About iOS version in iPhone 6 to update 13