You need undo the big bolt holding the bottom of the shock - it’s pretty tight and is [edit] ignore the threaded advice ![edit]
Then the arm just drops down and you and you pull the spring out (standard springs need compression but lowered springs should just pull in out easily by hand as the space is much more than the spring height). Fit I used the jack to lift the arm to the correct height and do up the shock again - there may be a spacer on the bolt so note that and refit it.
Just check any brake pipes and wires are not stretched as the arm drops in case they caught on something.
I was very nervous doing it first time but after about 5 goes it’s dead easy, felt like a formula one pit mechanic..