Adjust your established boundary any time.

Right click the project boundary on the map and select "Edit project boundary"

or

Tap Spacebar to open Command Line and search for "edit project boundary"

or

Click Edit Project Boundary in More Tools

The boundary edit toolbar appears.

Edit the boundary using control points and extrusion handles.

Select map shapes and use "add to project boundary" to expand.

If the boundary has more than one shape, click the red X on a shape to remove that shape from the boundary.

Click “Clear” at the top right to start from scratch.

Click Save Changes to apply your changes.

You can't delete the project boundary. It's part of the project, so it has no delete option. The only way to remove it is to clear it in boundary edit mode:

If you haven't cleared the boundary this way, your project still has it.