Inherited used iMac and want to change user I'd to me but save his music etc?

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You want to change the username and the user password without losing anything. Is this correct?

If so, first you can change your user account name. The steps are a bit "difficult", meaning that you may damage something if you do not follow the steps, so be careful. See > http://support.apple.com/kb/ht1428 You can change the full name in System Preferences > Users & Groups.

Then, if you know the current password, you just need to go to System Preferences > Users & Groups, and press Change password

We may be able to go farther if we knew the iMac model and the Mac OSX version. A good start is to do "About this Mac" from your Apple menu, which gives this window:

Note the two red arrows; please tell us what those indicated lines say on your computer.