When editing an existing Offset Curve in a Sketch, the Offset Distance is not displayed on the dialog as an editable field.
With the Offset Curve dialog displayed, locate the Offset section of the dialog and see if "Create Dimension" is checked. When it is checked the Offset "Distance" becomes an editable field. When unchecked, it is not displayed.
If having entered the new offset distance it is decided that a offset dimension is not desired, simply uncheck the Create Dimension checkbox and complete the edit.
Notes and References
OS Version: 764SP1