The cast member ID's are automatically numbered based on the positioning on the Cast Member page. For example, if you're looking to change cast member 1 to Cast Member 12, simply click and hold the dragger icon to move it down.
Step 1 ⏤ Open elements
Hover over the Breakdown Features tab at the top of your project sidebar to reveal & click Elements from the drop-down.
Step 2 ⏤ Open element sidebar
Click the sidebar icon to toggle the element sidebar.
Step 3 ⏤ Select cast members
Click Cast Members from the element sidebar.
Step 4 ⏤ Pick a cast member
Hover over a cast member to reveal the dragger icon (≡).
Step 5 ⏤ Reorder cast members
Click and hold the dragger icon (≡), then drag the cast member to a new position. Release the mouse once the member is in the desired position.
Now that you know how to update cast ID numbers, here are some other helpful resources: