You will need to be a license owner to add a new owner, either in addition to yourself or as a replacement.

Log in to the customer portal using your Redgate ID. 

Click on the license you wish to add an owner to: 

Click on the cog in the top right-hand corner of the license screen: 

A new window will pop up titled Manage owners and admins. Click  +Add:

Enter the email address of the new owner, click the Owner tab then click Add.