From the Seeding or Run menus, click on the Adjust or Preview button for the Event you have selected. Make the changes that you want to make and if you end up with an empty heat, click on the Delete Empty Heat button on the left side of the screen. MM will ask for confirmation and then delete all empty heats.
For example, you may have started with 5 heats but because of scratches and by combining heats, you may end up with heat 3 being empty. Just click on the Delete Empty Heat button and MM will adjust the heats so that you end up with 4 heats numbered 1 through 4.