Replacement pressure gauge

[lesf]

Warm flow agentis external oil boiler E21C has a faulty pressure gauge on the expansion tank, I have a new replacement but the plumber has disappeared
What do I need to do to replace it myself. I'd do I drain the system, can I just release the pressure, thanks

 

[Exedon]

As its sealed system just drain enough to get rid of system pressure.
Have new gauge ready unscrew old and replace.
Always handy to have a couple of old towels or similar to mop up any leakage.