Currently it is not possible. The UI doesn't contain controls to change the default interval and when you change the value on the widget it is not saved to server side.
Theoretically it should be possible to export the dashboard (in Explore / Content), edit the XML file and import the file back. But we have identified a bug which prevents importing dashboards back to the VMAN application - the performance widgets get unconfigured after import.
This mean that with the current version (6.1.1) it is not possible. It should be fixed in some future version of VMAN.