new computer with 8.1, pop up says I must authorize computer to download previously purchased music. How do I do this?

Hello bsmith1039,

Thanks for visiting Apple Support Communities.

You can use these steps to authorize your computer to play and downlad iTunes Store purchases:

1. Open iTunes on the computer you want to authorize.
   
2. Choose Store > Authorize This Computer from the menu bar.
   You might need to show the menu bar to see this option in Windows. In earlier versions of iTunes, you can access this option from the Advanced menu.
   
3. Enter your Apple ID and click Authorize.

From:

Authorize your computer in iTunes - Apple Support

Best Regards,

Jeremy