Whether you want to split or merge fields, first you need to archive all fields for which you want to change the surface and then create new field(s) with new surface. Archiving of old fields is necessary because those fields were previously used (linked) in certain crop productions and they have some yield and usage recorded. To archive a field, click on the box on the left side of it -> Manage-> Archive.
To see video, how to archive a field, click HERE.