Distributions in the Basic or All categories are read-only and cannot be changed. However, after you have copied a distribution or used it to define an assumption, you can modify it in a number of ways.
To modify a distribution:
Create a distribution as described in Creating New Distributions and Copying and Pasting Distributions.
Right-click the user-defined distribution and choose Modify (or click the distribution and choose Edit, then Modify in the Distribution Gallery menubar).
A prompt opens as a reminder that you are modifying a distribution and not defining an assumption.
Click OK to display the distribution in a window that looks like the Define Assumption dialog.
Modify the distribution as if it were an assumption, following the instructions in Defining Assumptions.
When you are finished, click OK.
The modifications are saved with the distribution in the category and no assumption is created in the spreadsheet.