There are a couple of ways.
In the Toolbar, in the Bitmap section, there's an eraser tool. It looks like a pink box, an old-fashioned pink eraser. Select that. In the properties panel, you can set the size, shape, edge (fuzziness of the edge), and opacity of the eraser. For getting in very close, you might need to set the size down to 1 or 2.
You can also use the magic wand selection tool. When you select that tool, adjust the tolerance as needed. The tolerance is how similar the surrounding colors need to be to the one you click on. Then, click around your text to select the colors you want to remove and press your Delete key.
In either method, Ctrl-Z (undo) is your friend.
It sometimes helps to make your canvas color something different from your text or the background you want to remove. As the pixels are erased or deleted, the background color shows through and you can see better what you're doing. (Modify>Canvas>Canvas Color...)