I want to replace the blue part of the image with red. I've tried this using a combination of the
Magic Wand tool and the
Paint Bucket but that doesn't work well.
Can somebody please help me out?
Create a new adjustment layer. Click the Icon the lower red arrow is pointing to! Then you can adjust the hue slider, (the top red arrow). Keep in mind this will not always be the case. It works in your scenario because blue is the only actual color and the rest is black and white.
There are also a number of other ways to accomplish this. However this is likely the most efficient in your case.