When you create a new measurement, you're able to choose between three different measurement types:
However, once this choice is made (either by saving, or by creating a measurement underneath its heading), the measurement type is locked, and the options will be greyed out.
Measurement types are locked to prevent changing existing measurements to a different type that isn't logical (EG: trying to change existing area measurements into points, or vice versa).
If you want to change an existing locked measurement type, you'll need to delete it and create a fresh one.
How to: Make linear & area measurements.