Is there any way to change 'public' to 'private' in Qt Designer?
There isn't in QT Designer.
You should subclass it as Wysota suggested to you. So you have to use code to accomplish this.
I don't know exactly what you want to accomplish, but here are some suggestions :
You could use a private member variable with public functions to set and retrieve it. If you don't need the lineedit anymore, you can delete it.
You could set the read only property of the lineedit to true. so your users can't change it.
Cheers