I need to edit text in a jpeg file. How do I do that?

Unfortunately you can't. A JPEG is a flat raster (pixel-based) image file which has no awareness of "text" inside it. You could try to erase the text and graphic under it and then recreate the text, but it's best to do so using a mastering format such as PSD (Photoshop) or PXM (Pixelmator), so that you can export as JPEG but keep your layered master for later editing.

Matt