.mpk files are just zip files. So you can open them with an appropriate zip tool (e.g. 7-zip).
If you want to work on a widget, the better approach is to locate it on github and make your changes in a fork. Then you can also make use of the build tools to automatically generate a new .mpk file from your changes.
Rename your ChartJs widget from .mpk to .zip file. Then extract the documents from that zip file and make your modifications. After completing your modifications send those files to zip file. After that again rename that from .zip to .mpk . Then you can use this .mpk file.