Open the back of your phone and remove the battery. You’ll see the MicroSD slot to the right of the SIM cards. You can use MicroSD cards of up to 64GB (of any of the formats available in the market: SD / SDHC or SDXC).

After you insert the MicroSD card, put the battery back in and put the back cover back on. Press the On button to restart your phone.

Some users have reported problems reading microSD Cards (Type SDXC) of 64 GB capacity. 

Some SDXC type cards come in a certain format called exFAT. This is a proprietary format as you can read in the wikipedia source. 

Fairphone does not support this format, but there is an easy way to solve that. You can reformat your SD Card to a format called FAT32 (or other formats). There are multiple sources online to help you out with this. Here is one.

It is also advisable to re-format your SD card directly from your phone:

• System Settings
• Storage
• "Erase SD card"