If your static text is called handles.exp_counter, then that's going to be a problem because the name handles.exp_counter already refers to the counter variable (initialized to 1 in the OpeningFcn and incremented by 1 in Next_button_Callback).
Instead, call your static text something else, e.g., handles.text_exp_counter, and then set its String in the OpeningFcn and in Next_button_Callback (i.e., whenever the counter handles.exp_counter changes):
function franck_guide_OpeningFcn(hObject, eventdata, handles, varargin)
% ...
% ...
handles.exp_counter = 1;
set(handles.text_exp_counter,'String',num2str(handles.exp_counter));
% ...
function Next_button_Callback(hObject, eventdata, handles)
% hObject handle to Next_button (see GCBO)
% eventdata reserved - to be defined in a future version of MATLAB
% handles structure with handles and user data (see GUIDATA)uiconfirm(handles.UIFigure,'Are You sure?','Confirm Close',...
handles = guidata(hObject);
new_counter = handles.exp_counter + 1;
if new_counter > numel(handles.v_thickness_1)
msgbox('Experiment is Done!','DONE');
return
end
f = msgbox('Operation Completed','NEXT');
set(handles.text_exp_counter,'String',num2str(new_counter));
handles.exp_counter = new_counter;
% delete the old patch and create a new one:
create_patch(handles); % this calls guidata(), which stores the new handles.exp_counter value
(doing num2str is not strictly necessary)
