This is the bulk edit tool: https://www.geotab.com/mygeotab/sdk/bulkEditTool.html. You can use this tool to bulk edit devices but it is also useful for removing offsets from vehicles. Click on the link and you will be taken to a login page. For the server number you can enter my112.geotab.com and enter the database that the device is in. Enter your credentials and sign in. Click on Device and select the appropriate properties to edit.
In this case select the Odometer Offset(KM) property. Enter in the serial number and an offset of 0 separated by a pipe. Click "Analyze" to ensure your input is correct and then submit the changes.