Steps
Select My Apps in the top menu
Select Contact Forms
You arrive on the Contact Forms page
Press the cog icon next to the name of the Contact Form
A menu appears. Press Edit.
You arrive on the Edit Contact Form page
From here you can
Create a new field (press Add new field)
Find an existing field to add to the form
Remove fields from the form
Reorder fields on the form (press the Up/Down arrows next to the fields in the Your Contact Form section on the right)
Change the input type for a List field, e.g. from Single Choice, to Limited Multiple Choice or Unlimited Multiple Choice
After making changes, scroll to the bottom and press Save
You are returned to the Manage Contact Forms page