Change top Constraint of wall

Dear community

Top Constraint of a wall is a parameter. The value of the parameter is a Level. So far so good. Although, i want to remove all the top constraints and change them to ‘unconnected height’. I don’t understand how Revit declares this value to the wall. It’s not a string, not a level, it is something:

Any ideas how i can change all walls to unconnected height and remove the Top Constraint?

Thank you!

Thanks Bjorn.