You can edit a Contact Form that you are using, even if it is already being used by your contacts.
If you remove a field from a Contact Form which is in use, it will remove the data entered for the individual contacts who had data in that specific field.
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 button shown by the red arrow)
Find an existing field to add to the form (Use the Search bar and/or the Field filter checkboxes in the Find a field section. Do not press the Enter key. Search results will appear to the right of the Field filter checkboxes)
Remove fields from the form (press the X symbol shown to the right of the field in the Your Contact Form section)
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