Problem: When I try to change the priority of a process in Task Manager, it gives me an error:
"Unable to change priority. The operation could not be completed. Access denied."
Solution: This issue plagued me for a few minutes and saw quite a few folks have issues about this on the Internet. The first issue could be that you don't have administrative access (admin) on your PC. Assuming that's not the case, since you're smarter than that and even searched for an answer on the Internet, the probably issue is that Task Manager needs to be re-run as admin. Click Show processes from all users or Start -> Run -> taskmgr, right click and run as admin.
Why do I want to change priorities of a process? You may want to give certain programs higher access/priority vs others. Let's say you're working on an extremely important Excel workbook or doing video or image editing, you can set other programs at a lower priority and focus on your most important tasks. This "speeds" up your work, but in reality you're manually adjusting and tweeking what works best for you and not what Windows thinks should be done.