Use the ItGupLine routine to access and update a specific row in a table:
void SAP_API ItGupLine(ITAB_H itab, unsigned line);
This routine returns the address of the requested line (specified by line). ItGupLine functions just like ItGetLine, except that you may update the data at the returned address.
The corresponding ABAP operation isRead Table... Index..., Modify...
This function is defined in SAPITAB.H.
handle of an internal table
number of the desired row, row numbers start with 1 similar than Sy-Tabix in ABAP