Instructions
Speak with Otamamon in Shine Plaza to look at his stock of Farm Goods for upgrading your Digifarm. Purchase a Terrain Board from him -- any type of Terrain Board will do, but you should get something that will help your Digimon. Terrain Boards correlate to Digimon types, such as Dark and Holy.
Install the Terrain Board in your Digifarm by accessing the PC at the Tamer Home. Use the PC on the left side of the desk to access the Digifarm. When the Digifarm main menu is up, you are given options and instructions for installing any Farm Goods you've purchased from Otamamon. Follow the instructions to install your Terrain Board.
Return to and talk to Otamamon again to purchase upgrades for your Digifarm. DPU is measured in gigahertz and correlates to how many Digimon you can have on the farm. Memory is measured in gigabytes and correlates to how many Farm Goods you can install. The maximum DPU is 8 gigahertz and the maximum memory is 32 gigabytes. When you purchase upgrades, Otamamon automatically installs them for you.