would I just say multiply the pixel value by 10
Yes.
Or do I say something like pixelintensity=pixelintensity*10?
You need to store the resulting value back into the array.
You can implement this task with two nested for loops with the inner one affecting one value at a time. Or, You can implement the task with one for loop that works on an entire row at a time. Or, you can implement the task with one assignment statement and no loops, using index vectors.
