it's quite easy, i used a tool that holds the spring down so you can remove the collets etc and do it without removing the head. you can get them from your local spare parts dealer. just remove the rocker cover, remove all the rockers but keep them together with the guides and nuts etc on a board marked with 8 sections, then use the tool to clamp the spring down and remove the collets, take off the tool and spring and then change the seals, refitting is the reverse of removal ( ala haynes )

i've prob missed summat out but it's late and someone will put you right