How do you remove colons : from form fields?

To do this, edit the form, then select the field you want to edit.  On the right hand side, you will see a Label.  Click on this and you can edit the Label Name (delete the colon).

You can go into the Form Editor and remove the colon from the Label of that field. Click on First Name, then in the lower right, you'll see the options for that field.

Check out my post on this. Most of the links should lead you to how to do this in more detail.
http://www.marketingrockstarguides.com/marketo-form-instructions-164/

Both are correct, just edit the label field.