You must deactivate a custom input form before you can edit it, delete it, or change the companies that it is assigned to.
To deactivate a custom input form:
Once a form has been deactivated you can:
After editing a form or changing the companies it is assigned to, you must click Activate to make the form available for use again.
Related Concepts
About Related Lists on Custom Input Forms
Standard Salesforce Related Lists on Custom Input Forms
Related Tasks
Assigning Custom Input Forms to Companies
Managing Related Lists on Custom Input Forms
Reference
Input Form Manager - Sales Invoice Fields
Input Form Manager - Sales Credit Note Fields
Input Form Manager - Cash Entry Fields
Input Form Manager - Journal Fields