My girlfriend just gave me her  old iphone with v 7.1.2 on it. Should I update to the newer 10.2.1 that it wants me to update to? Looking to use bluetooth for the h9 max.Thanks.

Well, we haven't heard of anyone having trouble with that specific version of iOS.  Typically, when people have had trouble with Bluetooth it has been with specific iPad versions, and those troubles persisted across iOS releases.  I don't think you should run into any problems upgrading an iPhone to the latest version of iOS.