I would be very carefull here, it depends on your skill level
anyway
1. Find the outside stop,valve
and see what sort of end it has
some are tap ends and others are unique square ended ones
get the proper iron to do this
2.then see if it works,, spray wd 40 and have a go,,,if it really goes badly..you will get a gusher ...when the tap body comes out
3. I have had loads ...over the years..make sure you know your local utility phone number
if it goes bottom up
4. Then when ur sure all water is off
just haveca go undo things and take it apart. Then take bits to your local friendly proper plumbers merchant ..get the right bits and off you go
5. looks like you might need a lead lock IMHO
Centralheatking