knowing a spefic pixel rgb value how to a find and replace it(the pixel) with another rgb value.
I wrote a script a while back which basically mimics GIMP's Select By Color tool, but permits you to specify the color directly rather than picking it from the image.
After running the script, you will be left with only those pixels that match your color being selected. You would then want to do a bucket fill (e.g., "Edit->Fill with FG color") to change the color.
Choose: Windows / Dockable Dialogs / Pointer.
A window appears that shows you integer values for RGB of the pixel under the pointer (the mouse.)
Zoom in enough that you can distinguish individual pixels and be sure the pointer is over the pixel you are interested in.
You can also zoom in enough that you can select an individual pixel and set its color by choosing from a color pallette.
If you mean "how do I use the keyboard to enter integer values for RGB values of a pixel", I don't know the answer.
If you mean "in a script how do I get and set the integer values for RGB values for a pixel given by x and y coordinates", thats a different question.