Define default value for an input field by changing it on stage (similar to how it is with sliders)

Are you looking to change the prompt in the text input box?  See the attached video. The "type something else" is the prompt the learner sees, and not related to the variable. In the case of a slider, the slider reflects the variable, or sets it. In a text input box, it only sets the variable, unless you delete the trigger. Then it does nothing.  If the variable has a value, that value replaces the prompt. If you just want to change the prompt, type in it.  There is little point in having a default value in a variable that is changed by a text input box, if I understand what you are asking. A default value is used so the variable can have a value until you change it. Changing it can't be done the way you want, but doesn't need to be. If the variable is blank, the text input box prompt will be used, and you can change that by typing in the editor.

Or do you want to change the default values for the variables? The default value is the value the variable will have if not changed. That is, until the learner makes an entry into it. Then the variable is changed by this trigger:

which is automatically inserted when you insert a text entry field. You can edit this trigger to attach the entry field to a different variable.