ModbusRTU slave

Hello, I want to learn how to save data in the modbus_array [2] in the slave arduino, and with the master arduino read the result depending on the value that the generated if gives it, 1 or 2, I can't read those values, I use the Modbusrtu library (smarmengol) and arduino ModbusMaster for the master arduino can someone help me understand how to save a value in an array of the slave's modbus THX.