Steps
Go to the Manage Contact Forms page
Select My Apps in the top menu
Select Contact Forms
Press the cog icon next to the name of the Contact Form
You arrive on the Edit Contact Form page
From here you can
Find an existing field to add to the form
Remove fields from the form
Reorder fields on the form
Change the input type for a List field, e.g. from Single Choice, to Limited Multiple Choice or Unlimited Multiple Choice
After making changes, press save
You are returned to the Manage Contact Forms page