It seems like you haven't gotten a response just yet. Answering the following questions may give others in the Apple Support Communities more information, so they can help you get a solution:
What shortcuts are you trying to change, and what are you trying to change them to? What happens when you try to change them (do you get an error message, for example)? What is the exact macOS version number, from Apple menu > About This Mac (example, macOS 10.13.2)?