I don’t have a nest but I did fit the wireless thermo and it’s just sticking in the correct wires…I’m doing this from memory but the key is having your boiler manual (downloaded from internet)
You want your boiler manual as these are either a small DC switched or mains and some boilers have both.… The Hive I’d guess wants the
low voltage DC one so you’d likely fry it connected to the wrong one…
A decent modern boiler probably only has low voltage…. I have a reasonably modern boiler but its crap!
However, your existing Wireless is almost certainly low voltage DC (but worth checking) so it should be a straight swap. If it plugs in on the boiler side of the wireless then even more likely its low voltage DC….