Q: how do I change my password on my ipod as I cannot access yahoo mail as the password has disappeared for some reason

How do I change or recover a forgotten Apple ID Password?

If you've forgotten your Apple ID Password or want to change it, go to My Apple ID and follow the instructions. SeeChanging your Apple ID password if you'd like more information.

The referenced article provides an alternate means to reset the password. If you forgot the answers to your security questions then:

Forgotten Security Questions/Answers

You need to contact Apple by:

1 - Use the Express lane and start here:

https://expresslane.apple.com

then click More Products and Services>Apple ID>Other Apple ID Topics>Forgotten Apple ID security questions.

or

Apple - Support -form iTunes Store - Contact Us

2 - Call Apple in your country by getting the number from here:
http://support.apple.com/kb/HE57

or          

Apple ID: Contacting Apple for help with Apple ID account security

3 - Use your rescue email address if you set one up

Rescue email address and how to reset Apple ID security questions

For general  information see:

Apple ID: All about Apple ID security questions