You can't actually edit text that's in place in an existing image if you don't have access to the original file and text layers
You can, however, replace text in an existing image, but it takes work, and it's totally dependent on your ability to remove the the existing text, how much room you've got to work with, ability to blend the background it was on, recreate the Font/Style, etc. Also, a problem with most web images is that they're typically low resolution (e.g., 72dpi), so anything you recreate needs to have a blur added to blend it and give it the same effect. I actually saw a video tutorial last month on text replacement (the author was putting his own text/name on liquor bottles in the same original text style, and you couldn't tell the difference) either on NAPP, Kelby, or AdobeTV, but so far I haven't found it again or I'd add the link for you.
Essentially what you'd need to do is select the existing text, and do a Content Aware Fill. Once that's done, you'd need to create your own replacement text and style it/use perspective tools/healing brushes/blurs as necessary to replace what you've removed over the filled background, and blend it into the existing image.
It's a lot easier in Photoshop CS6 with the Content Aware Patch/Move tools.
If you're new to Photoshop, it's probably beyond your level at this point - you need a good knowledge of/experience with Photoshop to use many of Photoshop's advanced features, and that takes time and practice. There are at least a dozen advanced features I can think of off the top of my head you need to know, probably more.
Here's some links to resources and tutorials that will help you learn:
Creative Cloud
http://www.adobe.com/products/creativecloud.html?promoid=JQPEQ
Adobe TV - Photoshop Channel - http://tv.adobe.com/product/photoshop/
Planet Photoshop - http://planetphotoshop.com/
Photoshop Killer Tips - http://kelbytv.com/photoshopkillertips/
Layers - http://layersmagazine.com/category/photoshop
National Association of Photoshop Professionals (NAPP) - Some of the best tutorials on the planet, and a great intro vid for PS beginners. The discounts on Adobe and other software/hardware are worth the subscription alone.
http://www.photoshopuser.com
Kelby Training - NAPP's professional course training site - http://kelbytraining.com/
Lynda.com - Professional Tutorial Courses - subscription (various plans) http://www.lynda.com
Lyndapodcast (YouTube) - http://www.youtube.com/user/lyndapodcast…