I have Unity on 2 computers and just go a new computer for Christmas. I know that Unity only allows you to have Unity on 2 computers.
How can I download a new copy of Unity on my new computer, and delete a copy of Unity on my old computer.
On Windows go to Help → Manage License → Return license
On Mac go to Unity → Manage License → Return license
and then you can use the same serial number on your new computer.