Since you have a flat file, you can't do anything unless you first separate the text from the background on a new layer.
It may be possible to select the specific color of the text and to paste the selection on a new layer (Ctrl J)
Be sure to delete or mask similar color parts outside of the text.
Now you can change your background.
You probably don't want to change it globally, only under the text parts. Maybe create an intermediate layer copy of the background. Create 'boxes' selections under the text lines. Select inverse and delete. Suggestions: fill with flat color, reduce contrast or saturation...
Sometimes you can work on the text layer (which is still a pixel layer...)
Select all (Ctl click on the thumbnail in the layer palette). The letters contours should show the marching ants.