From the Patient Dashboard, click Insurance Management in the Active Policies panel.

In the Insurance Profiles panel, deselect the Active checkbox on the Default Insurance Profile.
On the chosen Insurance Profile, select the Active checkbox.

Use the Patient Dashboard to change the Default Insurance for all future Encounters.
Use the Encounter Dashboard to change for one Encounter only.
Select the Insurance/Employer Management link in the Active Policies Panel.

Select the Active link (underneath the trash can) on the Insurance Profile that you want to make the Default.

Select the checkbox in the Default column.
Select Save [F2].

Encounters can be moved to the new Default Insurance if necessary.