The KK2.1 is a very popular flight controller as its cheap, works well, and does not need a PC to configure thanks to its built in LCD screen! This guide will provide links to the various manuals, and useful videos and links to help you get started with your KK2.1 flight controller.
Before you start, the most important thing is to make sure you connect the battery correctly on the KK2.1 boards as they don't have reverse polarity protection built in.
The first manual is the best one as it explains everything nicely. It also has a nice table at the end that you can print out to write down various settings
KK2.1 Instruction Manual V1.16S1 Pro.pdf (435.7 KB)
This second one is the manual from HobbyKing and has some useful diagrams
HK KK2.1 Manual.pdf (431.6 KB)
Lastly you might also want to have a look at the KK2.1 Quickstart Guide
KK2.1 Tuning Guide Video
This is a great video that walks you through the tuning process for your KK2.1 board.
Updating Firmware on KK2.1
What you will need:
Here is the video showing you how to flash firmware onto the KK2.1. Its a good idea to first setup the USBasp before you try to flash the firmware. Also note that this video is for KK2.0, but its just a matter of selecting the correct board. You can also download the latest firmware version here