I am currently applying for a promotion at my current job and have been here for 26 years. I haven't updated my resume of written a cover letter in over 20 years. Can you give advice on this?

Well first, congratulations! Asking for more responsibility and growth is always a big step! Applying is to be celebrated. Second, if you are internal, are you sure these tools are required? They may not be, so ask if there is a different process for internal hires.
If you need this materials, start by looking at the job description.
What are some key words and themes that are highlighted? How can you align your experience to date with what's required? Start by listing EVERYTHING you've done for company and everything relevant outside of it. Then scale it down to the core details needed.
If you are really struggling with format and technique, consider a resume/cover letter service to help give you a second eye and critical feedback. If you went to college, your career center might offer this type of service for free. Do some digging!