How to deactivate previous Slate Pro plugin.

Hi,

i have the Slate PRO license key, but i need to deactivated the previous one first. how do i deactivate the old ones? it says i still need to purchase a license eventhou i already entered previously.

this is the error message that i received.
“Looks like this key is already activated. It needs to be deactivated before you can use it.”

Please help, and thank you.