Thank you for your message. I am sorry you are having this problem.
As @Ten A says, upscale only works for images generated in fast mode. If the image was generated in fast mode, the upscale button in here:
If the image has already been upscaled, the upscale button us greyed out. If the image was not generated in fast mode, the upscale button does not appear.
If you only want to use one credit and get a full sized image, slow mode is better. Fast mode is often used if you are in hurry or just want to generate a lot of images quickly.
Slow mode you use one credit for full sized images. Fast mode you use one credit to generate and one credit to upscale each individual image.