Matlab – Editing checkboxes in a Matlab GUI

The simplest way is an anonymous method passed into Label.Invoke:

// Running on the worker thread
string newText = "abc";
form.Label.Invoke((MethodInvoker)delegate {
    // Running on the UI thread
    form.Label.Text = newText;
});
// Back on the worker thread

Notice that Invoke blocks execution until it completes--this is synchronous code. The question doesn't ask about asynchronous code, but there is lots of content on Stack Overflow about writing asynchronous code when you want to learn about it.

When you first plot the image with imshow, have it return a handle to the image object it creates:

A = (the initial matrix of image data);
hImage = imshow(A);

Then, to update the image with new data, try the following instead of calling imshow again:

B = (modification of the original image matrix A);
set(hImage, 'CData', B);

Using the set command will change the image object you already created (a list of image object properties can be found here).

Alternatively, you can also add additional parameters to a call to imshow to tell it which axes object to plot the image in:

hAxes = (the handle to an axes object);
imshow(A, 'Parent', hAxes);

EDIT:

Addressing your additional problem of sharing GUI data between functions, you should check out the MATLAB documentation here. As noted there, there are a few different ways to pass data between different functions involved in a GUI: nesting functions (mentioned on SO here), using the 'UserData' property of objects (mentioned on SO here), or using the functions setappdata/getappdata or guidata. The guidata option may be best for use with GUIs made in GUIDE.