To make this example easier, let's assume you want to edit a track called Rally n142. (I don't think there's such a track...but the instructions work the same for the one you want to edit.)
1. Unpack the POD - say rallyn142.pod. C-POD works fairly well for this:
Good instructions can be found on the page as well.
2. Remember to unpack it to your traxx folder (/art, /models, etc), or just change your traxx settings to look in the folders you extracted the pod to.
3. Traxx: File >> Clone TrackEd Track. (Actual wording may vary..I forget exactly what it says.) Look at the folders created when you extracted your pod... in the WORLD folder you should find a file with a .sit extension. RALLYN14.SIT. Open it up.
4. You now have an editable version of the track. Might as well save it as a TXX file now.
5. Make the changes you want. This works for stock tracks too.
6. Build the new POD file as usual. Might want to save it as something other than the original file - example rallyn142_edit.pod - so you can tell your version from the old one.
7. Enjoy the results in the game. Remember that you should use the original version of the track if playing online, so you have the same version as everyone else to keep it fair.