If you mean literally then cut or rip off the tag and replace it with yours, make sure the thread matches the shirt.
If you are asking where to get tags, texlabels.net is great. Quick response time, fairly cheap, and good quality. I've gotten both printed and woven labels from them and their turnaround time is about 10 days. Click this link
http://viewmorepics.myspace.com/inde...ndID=343491399 , and the second album cover is a picture of printed tags: they look fake but they are real. In terms of having the labels put in, go around to your local screen printers and they should have an idea. Locally, here in San Jose I only know of one place but the internet also has plenty of places. Another thing is to tear or cut out your tags - I recommend the latter - and then have tags printed on. Hope I was of some help.