Here is a demonstration video that goes through this process (Change part name and definition.mov, right click this link to save the movie to disk for better viewing):
Here is a DeviceEditor .xml saved design file resulting from saving after the demonstration above (Change part name and definition.xml, right click and save link as to save the design file to disk):
Changing a part's name:
With a part selected (active) in the design canvas, it is possible to change the name of the selected part in the panel on the right side of the DeviceEditor display. Change the "Part Name" field to the name you would like to give to the part, and then press the "Save" button. Note, if you do not click the "Save" button, your changes will be discarded. This is necessary, because DeviceEditor needs to check if you have entered a valid name for the part (e.g. the part name can not contain certain special characters).
NOTE: with a part selected (highlighted in light blue in the design canvas) it is possible to name the part even if the part has yet to be mapped to a sequence. Parts that have been named, but have yet to be mapped to a sequence, are colored with a red outline on the design canvas.
Changing a part's definition:
With a part selected (active) in the design canvas, clicking the "Change part definition" button in the panel on the right side of the DeviceEditor display will pop-up a "Specify Part Definition" dialog, which will allow you to change the definition of the part. In this dialog, you can change the name of the part (an alternate method to the one described immediately above), the portion of the part source sequence that corresponds to the part (e.g. toggle between "Whole Sequence" and "Specified Sequence"; and adjust the "StartBP" and "StopBP"), whether or not the part is located on the complement strand of the part source, and the "Five Prime Internal Preferred Overhangs" and "Three Prime Internal Preferred Overhangs" settings for the part (for more information, refer to the "Target part order list file" page in the j5 user's manual).