Our newest recommended method of upgrading and modifying firmware for the Ender 5 is now with Visual Studio Code and PlatformIO. It's a much easier and simple platform, and we prefer it over the Arduino IDE for most situations. Arduino is still acceptable, we are just sharing this as our 'recommended' method now.

1. Download and install Visual Studio Code.
2. From Visual Studio Code menu, install the PlatformIO IDE extension.
3. Once everything required is running, download and extract marlin source to a folder.
4. From the Visual Studio Code menu, choose Open Folder. Select the folder in which the platformio.ini file is located (generally the Marlin root folder, where other folders are located). Use the 'Open Folder' command and not 'Open File'.
5. Look for the Configuration.h inside the 'Marlin' subfolder and make the required changes or updates. You may also need to modify Configuration_adv.h depending on your requirements.
6. Save your files after editing them, and when complete go to the Terminal > Run Task > Build. It will take several moments to compile and create your firmware, if there are any errors, you can report them here or Marlin's website for support.
7. If the code compiled correctly and without issue, you will see a message that reads something like ' ===== 1 succeeded in 00:00:23.801 ===== '
8. Connect your printer via USB cable to the computer. (You must have boot loader installed prior to this)
9. Go to the VSCode menu again and find Terminal > Run Task > Upload.
10. If the upload to your printer is successful you will find another message that looks like '===== 1 succeeded in 00:00:23.801 ====='

These are the basic instructions to modify and compile/upload your new modified firmware to your printer, and we will be glad to help you if any issues arise!