You are talking about two different things.
To <u>remap</u> a model means you want to use a specific part of a texture. It may be just slightly different than what it already is, or it can be a completely different part.
To <u>replace</u> the textures a model uses means you tell it to use one texture in place of another.
Replacing textures is easier than mapping. Since you say you have a new texture, then let's begin with replacing. To do that, the easiest way is to use binedit. Open your model in binedit, and on the texture menu select replace
<center><img src="http://binedit.com/help/images/menu08.gif" width="143"></center>
Then just tell it what textures you want to use (the ones you made), click okay, and voila, it's like magic.
More info here
http://binedit.com/help/replacer.shtml
If you need mapping too, that'll take a bit more to explain... and we'll probably find you a couple help pages to start off with.