You can open an existing mashup from Dev Hub.
Do the following:
Open Dev Hub.
See Launching Dev Hub.
Select the mashup from the list.
The information dialog is displayed.
The mashup opens in Mashup editor.
You have now opened an existing mashup and you can continue editing the files.
If you open a mashup created in an earlier version of Mashup editor you will be asked if you want to convert the mashup to a new format.
If you wish to convert your mashup to the new format, click Convert.
If you chose not to convert your mashup to the new format, click Cancel.