It depends. If the existing tool is developed using GUIDE and the GUI components are saved in a .fig file, then you can use GUIDE to open that .fig file and modify the GUI components directly.
But the tool could be developed and saved entirely in an .m file, using uifigure(), uibutton(), uicheckbox(), etc. then you have to read the .m file and find the places where the positions are set and modify it.
BTW, Warning: GUIDE will be removed in a future release. Use APPDESIGNER instead.
