The switch of the new GE thermostat (i.e. terminals 6 & 7) below...
......goes to the boiler terminals marked with the same 'switch' symbol. [The diagram below is from the following page of your boiler manual to the one you posted the screenshot of]
If there is an existing room thermostat connected to these terminals it is disconnected, if there isn't an existing thermostat and the terminals contain a bridging link then that is removed first.
Finally the new thermostat requires a L and N to terminals 1 & 2 to provide it with 230V operating power. This should come from the same L and N as the boiler, so that they share the same 3A fused supply.
If the wiring terminals are located inside a room sealed part of the boiler (some are, some aren't) then it's not a DIY job and requires a Gas Safe engineer to make the connections so they can test it to make sure that the boiler is properly sealed up again afterwards.
If there is an existing room thermostat then it may be possible to simply replace it with the new one, but we would require details of the existing thermostat and its wiring to advise further.
