Hi Simon, There is two ways you can edit the widget
1) As widget is in mpk format ,you have to extract it and there you will get the js file ,which you can modify to accommodate your changes and you have to zip it again to make it mpk
2)recommended way is to change the files of widget in their GitHub repo and make a contribution over there , so that your changes will come as official release ,also you will get credit for that.
here is the github planner widget
Good luck!