1.install the Mission planner, you can download from here, https://firmware.ardupilot.org/Tools/MissionPlanner/
2.select the right COM port and 115200 baud rate.
the port cannot be seleted with AUTO or other. If you use USB to connect, baud rate should select 115200 , not other.
If your PC cannot recognize the port. First check your USB line, make sure it can work well. Second if the issue also happened, you should install the PC’ driver for the APM. driver can download from here,https://firmware.ardupilot.org/Tools/MissionPlanner/driver.msi
YOU can download it to install it on your PC. Some WIN7 cannot install it sucessufully, maybe you can change other PC system, i recommend the WIN10 system.
If your driver is like this, you should install the driver to your PC.
If your PC do not have the port, you should change other USB line.
3.Select “install Firmware Legacy”, do not select “Install Firmware”,because “Install Firmware” is used for PIXHAWK,not APM.
Make sure you can see the Version number,if you can not see the version number,it means that your mission planner cannot connect 3DR firmware service, you can not install firmware online.
4. Do not upload latest firmware (for example 4.0.3) offline for apm board.
Due to the limited memory space of the APM board, the latest firmware cannot be uploaded. If you use the mission planner to flash online, it will automatically help you to upload the firmware suitable for the APM board.For example, the copter firmware for apm is 3.2.1 (mission planner will help you upload this version),the latest version is for PIXHAWK , if you upload offline above 3.2.1, it will clear the bootloader of chip, when bootloader is broken, apm cannot work, also cannot upload new firmware.
APM 2.x (APM 2.6 and later) are no longer supported for Copter, Plane or Rover. The last firmware builds that fit on this board are Copter 3.2.1, and Plane 3.3.0, and Rover 2.5.1.
So, upload firmware online at mission planner, if you upload offline, make sure the version is not above 3.2.1(copter, for example)