If you have reached the limit of the number of computers which can be activated by your license, you will need to release the license from one of the computers it is currently activated on before you can activate another computer. This can be done by going to Release License under the Help menu in Geneious.
If you are unable to release the license from a computer where it was activated because you no longer have access to that machine (or the computer was reformatted), please contact support for assistance.