Click the Registration categories step under the right column Event set up
Edit all price capacities that are higher than the event's desired capacity to be equal or lower than the event's desired capacity by clicking Edit next to Price restrictions and changing the number in the Capacity field (if your event has public capacities, then those numbers have to be lower than their capacity counterparts, so change those if they will be higher than their desired capacity counterparts) and clicking Save.
Now repeat this type of change for the registration categories by clicking Edit next to Category restrictions and performing the same changes.
Go to the Restrictions and settings step and change the event capacity (if there is the public capacity, make sure it is lower than the desired event capacity before changing the event capacity)
Note: If you change a price's capacity/public capacity to lower than the current registrations for that price type, that price type will be sold out and will show that sold out status on the registration form if it is both available to everyone in its price restriction and is open for registration.
