To edit the date that the service is due or the amount of kms left in the service, select the vehicle from within the Service Manager and then select Vehicles Service Plans. 

Here, you can change:
The date under Next Service Due
The kms under the OR Due In 

Just click on what one you wish to edit and change.

Service Manager is permission specific, please contact your admin user or Coretex Support if you do not have access to this.