From the FAQ: "You can use VEGAS Pro (14.0 and later) simultaneously on 2 devices. There are 2 ways to deactivate a device so you can activate a different one:

Visit your MAGIX account > My Products > Show More Details > Deactivate. This option can be used only once a month.

From the VEGAS Pro Help menu choose Deactivate the Software on this PC. This option can be used as often as you wish. An internet connection is required."

Use the MAGIX account option to deactivate (especially if you do not use your old computer any more). the "My Products" section also has the link "Installation program".

(As in the two comments here.)