A. Select the Top viewport and click the Maximize Viewport Toggle button on the bottom right of the screen. Rotate the viewport until you see the inside of the rim, and select the 3 edges as shown in the following image:

B. Click the Loop button.

C. Click the Grow button two times. Click the Shrink button. You should now have the following edges selected:

D. Press B to switch to the Bottom viewport. Press G to get rid of the grid. Scale down the selected edges on the Y and X axis (simultaneously) to make the inside of the rim flat again.

E. Click the Maximize Viewport Toggle button on the bottom right of the screen to see all viewports again.