I am trying to change the background color of this photo from yellow to white however this is not text that i've added via photoshop, it is text that is already embedded in the image. Can I somehow change the background to white?
You can use a simple Levels adjustment...
Image > Adjustments > Levels
Move the right (white) arrow to the left so it's inside the big histogram indicators, then move the middle (grey) slider to the right to darken the mid-tones a bit. Moving the left arrow (black) to the right will darken the dark tones.
These are merely the basics, play with it until you are happy.
Afterwards, selecting everything except the seal and using Image > Adjustments > Desaturate can remove any slight yellow left behind around the type due to anti-aliasing.
Desaturate removes the hues and makes everything greyscale.
If the background image is a "pure" consistent color, you may be able to use the color replacement tool as referenced in this web site. The background color doesn't have to be spot-on pure, as there is some "fuzzy" allowed for tolerance.
- Image > Adjustments > Replace Color
- Tap in the image to select the color to replace
- choose the eyedropper with the plus sign to add to the selection
- Tap on any areas of background to change them
- Hit OK.
The linked site has images and some expansion of the above steps.