If you already have one or more Commands defined on your site, you can edit them if necessary. See the JSON Command Definition for information about what you can do in the Command.
In the XNAT UI, select Administer → Plugin Settings
On the Plugin Settings page, choose Images & Commands. This screen is organized by docker image, with a table under each image listing the Commands that use that image.
Find the row for the Command you wish to edit. Click the button with a pencil icon in the Actions column to bring up a JSON editor.
The editor window will be populated with the Command JSON definition.
Once you have made your edits, choose Save Command to save and close the window.
It is good practice to change the version number every time you edit the Command. However, this is not required.
If the Command is published in a publicly-available repository, consider contributing your edits back to the original. That way everyone who uses this Command will benefit.