BMW 3 Series 318i E46 manual petrol , 2003, (N42 engine 1995cc)

Hi all, I am new to all these forums and such like, and to my new BMW car, so I have a lot to learn, and this is my first ever posting.
The valve stem rubber seals on this car need replacing, what I need to know is do I have to remove the head , or can I use the compressed air method, or rope trick to hold up the valves .?
Would using these ‘s methods cause me a problem with timing ?,
Other’s have said that by using the compressed air method or rope trick that the timing is not effected, but would be effected if the head had to be removed…however!…nobody I know has worked on this bmw n42 engine , therefore I am hoping that some of you guys might have , and can offer me much appreciated advice.