You can edit a form that has already been created and published. To do this click the Open button next to the form, add, remove or edit the fields, save and Publish again.
Jobs that contain this form historically will continue to show the old version before edits. Any new job you create and attach the form will contain the new version.
If you need to add the new version of the form to an old job, edit the job and reselect the form which will attach the new version.
Even if the form has the same name the system will add the 'new' form with the changes you have made to it.
Then simply Save the changes and the new form can be accessed through the Forms section on the job.
Custom form builder
How to create custom forms
Adding forms to jobs