Whether you want to split or merge fields, first you need to archive the old fields whose surface area is no longer accurate. Then, you can create a new field(s) with the updated surface area.
Archiving old fields is necessary, because those fields may have been previously linked to a crop productions and have some yield and usage data associated with it. Archiving the field will save that data.
To archive a field(s), click on the check box(es) to the left of each field, then click Manage → Archive.
To learn more about archiving a field, click HERE.