Selling my old computer, must I choose deactivate menu from cs6?

Are you assuming CS6 will work on the new computer, and you won't need to get up to date with a subscription? May be a big assumption. 

For security reasons, you should wipe your hard drive clean and reinstall the primary operating system.  You don't want ANY sensitive data to fall into the wrong hands.

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or must I choose the deactivate menu in adobe cs6 first, uninstall the software, then install on new windows 10 with my own license key??

By @LLI

As far as I see you will only selling your computer and the CS 6 License you will used on your new computer. Right?

If yes you have to deactivate CS6 first and then uninstall. The best is to use then the reset option of Windows to wipe out all personal data.