Instructions
Go to the market and talk to the farm goods shopkeeper. In Shine Plaza the market is directly across from the teleporter that leads to your home. The farm goods store in Shine Plaza is run by the Otamon (he looks like a purple tadpole) at the end of the counter.
Select "Modify," the middle option on the store menu.
Select the island you wish to upgrade.
Select "DPU Expansion" to upgrade the maximum number of Digimon that can occupy a single island. The default DPU is four, but this can be expanded to eight, allowing a total of eight Digimon of any type or level to occupy that island.
Select "Memory Expansion" to increase the memory on each island. Memory measures the amount of farm goods that can be placed on the island. Increasing this amount is important, as it allows you to place training machines to increase stats as well as fields that provide food for your Digimon.