Using Gimp, I created a design with circular lettering. I increased the font size, and now the text is pixelated (blurry, lighter in color than the rest of the image). How do I select the existing text and make it sharper
If the text is blurry/pixelated when you resize it bigger, then the text has been rasterized, and is no longer editable text. Rescaling text using the scale tool will rasterize the text, and so too will other transformations such as rotation, skew/distortion.
Editable Text layers can be seen in the layers panel. They have a T icon in the layers panel. If the icon is missing, then the text is rasterized and can't be edited as text.
If the text is still an editable text layer, then you can select it with the Text Tool, and change the point size.
There is no way to turn rasterized text back into editable text. However, if the text is still on its own layer, you could delete that layer, and retype the text and change the point size.
If you want to manipulate text, and keep it editable, then GIMP isn't really the best software for that. GIMP is a raster image editor, essentially for editing photographs. It would be better to use vector image editing software such as Inkscape (which is also free) for the kind of work you describe.
For example in Inskcape, you can put text on a curved path, and it is still editable as text.