Welcome to the SW community.
Are you editing a dimension in a sketch or in a drawing?. If it is in a sketch, and from what you describe, it sounds like the dimension is driven, which means another dimension is driving in it's place (defining the geometry to be constrained.) Change the dimension from driven to driving (RMB) and correct the conflict. If you are in a drawings environment then the 2D geometry that you are dimensioning is driven by nature since parts/assemblies drive the shape of the drawing etc.
Thanks for the answer. I am very much a novice at this 3D stuff. My forte so far has been 2D vector drawing in Corel.
You very well may have answered my question; but, I am just too ignorant at this point to know whether or not you have.
I'll let you know once I have learned enough to know.
Have you gone through the tutorials under Help in SolidWorks? That is a great place to start and get a good foundation for using SolidWorks.