How do I down load previous purchased software to a new computer?

If you bought it as a digital download through Adobe you should be able to log into your adobe.com account and go to your order history. You should find a link to download your software.

If you bought it on physical media you will want to re-install it from the disc it came on. Alternatively if it is the most recent software you can download the trial version from Adobe.com and then register it with your product key which you would either need to still have on documentation or have registered with your Adobe account where you can retrieve it by signing in to Adobe.com.

It is probably also sensible to deactivate the software on the old machine if you aren't going to use it anymore.

This is one of the real benefits of registering your software with Adobe and buying direct with digital download, you will always have easy access to your software installers and product keys.