First, jpeg is a poor choice of format due to the vagueness of colour borders. Use a different format (e.g. png) if you can.
In any event, to change all occurrences of a colour to something else, first use the "select regions by colour" tool, adjust the "threshold" value to set the tolerance level, and click a point of the colour you want to change. That will select all areas of that colour. Then select the new colour, and use the "flood fill" tool to fill the selected area with the new colour.