Go to the wedding calendar, and select the wedding you'd like to switch and plan for. Tap on the wedding and a popup will appear with options to 'Delete Wedding', 'Edit Details' and 'Select Wedding to Plan'.

In the example below, choosing 'Select Wedding to Plan' will switch between Cindy Laure & Patricia Hayes wedding to Jenny Millers & Dave Rosevelt's wedding. 

Note: The currently selected wedding for planning will be indicated with a red header and blue tick (as shown above). In addition, the main planning view will display the name of the couple (refer to screenshot with purple box highlighted).