You can update the Rx buffer using the macro : CAN_RX_DATA_BYTEx(i), where x is byte number and 'i' is mailbox number.
For eg. If you want to initialize 1st byte of mailbox '0' you can write following:
CAN_RX_DATA_BYTE1(0) = 0x AA; // initiatized with 0xAA.
The same macro is used to access the received data.
Make sure that your code doesn't assign value to the macro after the reception of the data.
Hope this helps.
Please feel free to update for any clarification.
Thanks and Regards