You can use the magic wand tool to select the background/letter.
If the selection looks like this....
Hit Shift + Cmd/Cntl + I to invert the selection so it looks like this....
You can then remove the background by adding a layer mask.
Do this by clicking the 'add layer mask' icon located at the bottom left side of the layers panel.
(The layer mask can be reversed by clicking on it and then hitting Cmd/Cntrl + I to invert.)
You may have to clean up a few spots that were selected by selecting/clicking the layer mask and using the Brush Tool set to black
to brush out the remnants.
Then save the file as a PNG which supports transparency.
EDIT: Now, with the letter itself isolated from it's original background and saved as a PNG file, you can place it on any color background you desire. You will also notice that MrTom and I have both provided you with completed images. The above tutorial hopefully will help you and others in case there is ever a need to do this for yourself! Good luck!