Restoring previous versions of a dashboard

Luckily this feature was added last year. When you are in the report, under the Project menu, there should be a link called "Open previous version".

This should take you to a list of all the saves (not sure how far back the versions go), but I am assuming the change you are trying to revert isn't too long ago.

Select the version you want (it may take you a couple of tries to find), and I believe you can re-save the previous version of the report over the current version, making this the latest version (I just don't know if the versions between will still be accessible after saving, or if all changes are just reverted and the newest history is lost - I haven't used this feature much)