Editing Text

This is probably a really easy thing to do but... I've been making manips for a short time now but there's one thing I especially want to do that I haven't figured out. I've noticed some people are able to actually change the text in a picture (such as a movie poster) using the exact same font that was originally there. How do you do that?

I have Photoshop CS and Paint Shop Pro 8

Thanks in advance to anyone who can help!