How can I transfer a license to a client?
You just need to pass the purchase code etc. to them and remove all copies of the item from your PC etc.
Bear in mind that if you purchased the item then support/updates of the item will only ever be available to your account as this cannot be transferred.
It’s better to ask your client to purchase the item from their own account. So that they can manage it from their own account when any update will publish or will need support from the Author.