You can't really.
It is complicated in photoshop and there isn't a way to do it that doesn't destroy the background behind the text. You can try to clone and recreate the area behind the text once you erase it with the eraser tool if the background is one color but if it is a photo or complex design it is pretty much impossible. There's also a way to do it involving adding layers & such but that is way over my head.
Best thing to do is find the folks that post blanks and just add your own text to the blank design. Or google for pictures yourself (large or extra large images are best) and use those. There are some great Disney clip art and wallpaper sites out there..just google it and they will come!
That is why every time someone posts a design there is a throng of folks requesting it personalized...it's really hard to change something if you do not have (or did not design) the original blank creation