Using Arduino IDE to Build Code - Updating Correct Firmware

Sorry in advance - I know this is a very basic question, but I’m very new to this and I’m a bit confused. I have the APM 2.6, and I’d like to create my own flight mode. The problem is that I’m having a bit of trouble simply getting a compiler to work. I followed the instructions on (]dev.ardupilot to use a modified version of the arduino IDE on my machine running Windows. I followed the directions explicitly, but when I tried to ‘Verify’ the code in Step 7, I got a huge amount of errors. The first one notified me that ArduCopter ver3.3 is not supported on APM2 boards. There’s a link in the document for the firmware version 3.2.1. I downloaded it, but I’m not exactly where I should go from here. How to do I load the v3.2.1 firmware onto the board? Is the code that I downloaded from Git in Step 2 the newer, unusable firmware? Can I just replace the file I downloaded in Git in Step 2 with the v3.2.1 file I downloaded? Thanks in advance, and sorry for such a basic question. I’m new to this, and I’m stuck.