Pull them off with a pliars , push on the new ones by hand, and as Bandit says, apply some engine oil to the seal inner and the top of the valve guide. Prior to pushing in the valve ensure the area of the valve where the collet locates is perfectly smooth and also the top of the stem. They can, but not always have burrs on them from use.
Ok so done a little bit of cleaning but while sealing the stems with blu-tac ( to stop crap going down them) I noticed on the very top (cam side) of the stem the exhaust side they are a bit sharp as you say may have burrs on. Intake are smooth...How do i smooth these out??.
Also rather than attack it with dremal my train of thought is hit all the crappy carboned up bits with wheel acid (wonder wheels) unless the are rubber seals inside the valve stems this shouldnt be a problem should it.
Thanks Gaz