Hi Hadi;
You need to go to SEGW tcode in the backend system. and create a new project then redefine the odata service "HCM_TIMESHEET_MAN_SRV" for timesheet and implement/redefine the methods there to fetch the data according to your requirement.
Following are the steps screenshots to reach there.
SEGW tcode create new project
Right click on data model and redefine Odata Service
Enter the technical service name. In this case odata service for tiemsheet app
After import, generate runtime by right clicking on the project
ZCL_Z_HCM_TIMESHEET_MA_DPC_EXT is the class generated which will have the methods
More you can read in the below links
Extending an OData Service Using Service Builder - SAP NetWeaver Gateway - SAP Library
How to Guide: SAP Fiori Extensibility - Adding Custom Felds in OData
Regards,
Sarbjeet Singh