Change all pixels in one color to a different color

Generally changing a colour can be done two ways, if the shape is closed, you can fill it with the fill tool (the paint bucket shaped one) or you can apply a hue/saturation adjustment layer to another layer (by clipping it) or the entire stack of layers beneath it.
the fill tool

the adjustment layers

when using the Hue slider to change the colour (note the clipping curved arrow that points to the impacted layer there)

It would be best if you could show a screenshot of the whole window, so we can point out to specific tools and processes depending on what you are trying to achieve.