How can I edit the text in a JPG and PNG file and change a picture

test

1. Open your Image in Photoshop.
2. Double click the Background layer to make it editable.
3. With the Layer selected,  hit Ctrl+J a few times to make copies  -- good to have if you do something horribly wrong.
4. Turn off the Eye icons on upper Layer Copies.
5. Start with Layer 1 and the Eraser Tool to remove old text. 
6. Use the Text Tool to create new text.

Nancy

Can you post a screen shot.

Not easily.  JPG and PNG files are flattened, single layer files.  In other words, you'll need to destroy what's there and replace it with new content.   Depending on complexity and quality of image, this might be a lost cause.   Creating a new PSD (Photoshop document) file might be your best option.   The idea is to keep separate elements on their own layers so you can go back and edit them later if needed.   See screenshot.

Nancy