Tap the menu icon at the bottom right corner and choose Visible Calendars.
At the far right of the calendar you want to be the default - tap the triangle icon and choose the 'set default' option.
Create a new event and tap the 'calendar' option. Choose the calendar you want to be the default. This will take you back to your event editor - tap the calendar option again and you will see the calendar you chose previously is selected - tap the 'make default' option at the bottom of that page to set that calendar as the default.
*Default calendars will only affect new events. Events you have already created will not be changed.