I want to change the track number on an mp3 file so that they all play in order in iTunes. Is there a simple programme that I can use to do this?
You don't need any additional software. Just follow Taty's instructions and you should be fine.
You can right select a song, right click, choose get info, go to the 'info' tab and change the track number. If it doesn't allow you to do that it's because the mp3 is a read only file. Find it on your computer, right click on it, select properties and unclick the "Read only" tab. You should be able to change it then.
the mp3s are numbered either in the file name or in their ID3 tags or both.
I'm not sure what exactly you are asking. Do you want to know what part of the file you have to edit or are you asking for a way to batch edit several files?
Please have a look at the following MakeUseOf articles and if none of them answers your question, please come back and clarify what you want to do. Thank you and good luck! :)
How To Rename Music Files from iTunes the Easy Way:
4 Easy Ways Fix Music Tags & Organize Music Library:
How to Edit Your Music ID3 Tags with Audioshell:
Here are is another tool for batch editing mp3 ID3 tags:
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PS: Which operating system are you using?