If it is a calculated field as in a table calculation or a query calculation, you cannot change it.

Instead use VBA to update the field. Make a field called DueDate in your table, type Date/Time
Then depending on your setup, you will need to find a good time to fire the code, probably either in the after update of the issued date or the on current event of the form. To set the value of duedate to issueddate+30, then you can still change that value if it is something different. (Also would need to make sure the VBA code your using to set the date doesn't over-write the value that you changed manually, again that is going to depend on your setup choice)

Or just make a button next to the due date that says +30 or something, then in the code for that button make it set the value to IssuedDate+30, otherwise you have to manually enter it, that's probably the easiest solution. Then it will only automatically set the value when you push the button, otherwise you can manually put any date.

On click event, the isnull check just prevents you from accidentally over-writing things if it's already set.

If Not IsNull(IssuedDate) and IsNull(DueDate) Then
DueDate=IssuedDate+30
End If