Q:

How do I change each pixel

I am new to Python and I still don't know what exactly Qimage's pixel returns (it seems to a tupel of rgb or rgba -the lack of type declaration doesn't help) 
I want to grab each each pixel and change it.
newqim = QImage(imWidth, imHeight, QImage.Format_ARGB32)
for xstep in range(0, imWidth - 1):
    for ystep in range(0, imHeight - 1):

        pixelValueTuple = im.getpixel((xstep, ystep))
        pixelR = pixelValueTuple[0]
        pixelG = pixelValueTuple[1]
        pixelB = pixelValueTuple[2]
        copiedValue = qRgb(pixelR, pixelG, pixelB)

        newqim.setPixel(xstep, ystep, copiedValue)

Above is the provided code ,I thought I then iterate over that newqim, but I can't get a handle on how exactly I would do that in Python.
for xstep in range(0, imWidth-1):
    for ystep in range(0, imHeight -1):

A:

I'm not sure I understood what you want, but since you are new to Python, here go a few tips...
Unpacking
This code: 
pixelR = pixelR[0]
pixelG = pixelValueTuple[1]
pixelB = pixelValueTuple[2]

Is the same as:
pixelR, pixelG, pixelB = pixelValueTuple[:3]

If you are sure len(pixelValueTuple) == 3, then it is just:
pixelR, pixelG, pixelB = pixelValueTuple

PEP-8
A bit of nitpick, but python guys tend to be a little nazy about syntax. Please read PEP-8. From now on I'm naming variables according to it (camelCase for instance variables just hurt my eyes %-).
Range
You probably want range(width) instead of range(0, width-1).
>>> range(10)
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

>>> range(0, 10 - 1)
[0, 1, 2, 3, 4, 5, 6, 7, 8]

Now back to your problem.
width, height = 300, 300
im = QImage(width, height, QImage.Format_ARGB32)

for x in range(im.width()):
    for y in range(im.height()):
        r, g, b, a = QColor(im.pixel(x ,y)).getRgb()
        # ... do something to r, g, b, a ...
        im.setPixel(x, y, QColor(r, g, b, a).rgb())

Example
width, height = 100, 100
im = QImage(width, height, QImage.Format_ARGB32)

for x in range(im.width()):
    for y in range(im.height()):
        im.setPixel(x, y, QColor(255, x * 2.56, y * 2.56, 255).rgb())

im.save('sample.png')

Result: 

