If a custom attribute has not been used in a project yet, it may be edited. To edit, click on the Settings icon in the main left vertical menu panel and select Custom Attributes in the left column. Click on the vertical three dot menu for the specific attribute and click Edit.
Click the Edit button for the desired field, and click Next to review the other details. When all the details are reviewed, the Update button will be become available. Click on Update to finalize. If the custom attribute has already been used in a project, an error message will appear that will indicate it has been used and the changes cannot be made.