How do I restore a default report template?
Don't know a way through the UI, but I think you could do it through SQL.

The default reports appear to be defined in xml/templates/civicrm_navigation.tpl, which in turn gets copied into sql/civicrm_data.mysql.

You might make a new instance with the default settings by copying the definition:


Open sql/civircm_data.mysql and search for civicrm_report_instance. Find the INSERT statement which corresponds to your report.
Copy the INSERT statement into a text editor.
Change @domainID to 1 (unless you have a special/multidomain system; then adjust accordingly).
Open your MySQL tool (e.g. mysql cli or phpmyadmin) with an appropriate user+database.
Paste and execute the INSERT statement.


If you want to reset the existing report and know a bit of SQL, then you can massage the INSERT and produce an UPDATE for the existing report.