If you signed up for your Wagmo Wellness plan directly through Wagmo, you cannot swap one pet for another on an existing plan. A Wagmo Wellness plan is written for the specific pet enrolled at signup and is not transferable to a different pet.
If you'd like to replace one pet with another on your Wagmo Wellness plan, the workflow is to cancel the existing plan and enroll the new pet in a new Wagmo Wellness plan. See How do I cancel my Wagmo Wellness plan? and the enrollment flow in your Wagmo account.
If you'd like to add a pet to your household rather than replace one, you don't need to cancel anything. See How do I add another pet to my Wagmo Wellness plan?
If you enrolled in a Wagmo Wellness plan through your employer rather than directly, pet swaps may be available in specific situations. See Can I swap one pet for another on my Wagmo Wellness plan? (Employer plans).