James Stockdale

Hello, Firstly, apologies if this is a stupid question or if it has been answered before. I am editing a PNG file and would like to convert a defined colour into another. For example, in the screen shot below, I would like to change any pixel that is coloured light blue, into the darker blue. I can obviously do this on a pixel by pixel basis, but wondering if there was automatic way to do this to the entire file with is approximately 10k x 10k pixels. Thanks in advance!