Unless you have the original Photoshop file, you are out of luck. If you got it as an image like a jpg, it’s all just pixels. If you have the latest Ps version (from CS5x), you could try using the content aware fill and blot out the text, and then put some new text in. It depends on the image, though.
Do you have the source layered file? (e.g. PSD?) Or is it a jpeg /png / gif or any other flattened format? your question is not really clear?
If you have the PSD etc… it is easy to use the text tool: http://www.bittbox.com/photoshop/photoshop-101-edit-text-faster
Otherwise you will need to erase the text or draw a white shape over it etc… and rewrite what you want! Can we see this graphic?
Use the clone stamp tool to rub out the current text, then find a font that is similar to the original font if you want to copy that. Then create a curved path and type your text along that.
If you have photoshop - which you should get if you don’t - Simply colour out the letters using your paint tool. Then choose the elipse tool and follow this tutorial from adobe.