How do I merge two different fields in Geofolia?

To merge fields in Geofolia, when you have had two different crops in one field the previous year and only one in the current year >  CTRL left click and select both parts of the field replicating the view on the right (as below).

With them both selected go up to the tools  button at the top of the map and select the merge button

This will come up with the new field box where you can change the name, crop and details about the field as if you are creating a new field.

Click the OK button when you are happy with the details entered and this will bring the two halves of the field back as one, it is worth noting you can only do this when the two field boundaries are touching.