We don't support resetting a counter. Instead you can just create a new counter and assign it to the corresponding field.
If you really want to reset the existing one anyway, you can do it - on your own risk - the following way:
- Find the id of your counter in the database table "xcrscstcnt"
- Edit the table "xcrscounter" and set the column "counter" to 0 where the id corresponds to your counter-id.
Again the warning: If you accidentially reset the wrong counter, there might be serious inconsistencies in the system that might even lead to data loss!