Just take a 19 key, turn crank unclockwise until you see the "IT" sign thru hole.
Make sure decompressor has a play, otherwise turn crank one more round.
When you feel the very small play, take off cable.
Grease new cable with motor oil, put on place, make sure the play when tightened is not less than 1.5mm and not more than 2.5mm.
Not enough play = you'll burn your valve
To much play = decompression won't work properly.
With brand new cable, best play is 1.5mm
Check several times again when installed, by turning crank to IT several times, to see that the play remains the same.
Sure, decopression cable is easy to change but it can cause serious damage if not well done.