As Gene has already mentioned, the status of a link as visited or unvisited is determined by the browser itself and based upon your browsing history. The way to change the status is to edit your browsing history to remove any links that are currently "in the list" as this is what makes the browser denote them as visited. There are many ways to do this, the most arduous being going into browsing history and removing them by hand, and the most thorough going into the "Clear History" feature of your browser and wiping everything out for some set period of time. When I do this the "set period" is almost always "all time," so everything is wiped and all links become unvisited again.
It is generally a good idea NOT to become overly dependent on the status of a given link, as there will be times when you either need to use another browser and its status in that other browser does not match what it is in your usual one, or you will need (or want) to clear browser history and this will reset the status of all links to unvisited. I'm not saying that using link status is a bad thing, but you need to realize when it can change and, if you've taken the steps to make that happen, you need to be aware that your visited links are no longer visited links.
It's very difficult to selectively edit, at the individual link level, browsing history because the same link has often been visited literally hundreds of times over a period of months, and you're unlikely to have the time or patience to go through the entirety of your browsing history searching only for that link and strategically removing every instance of it.
Brian - Windows 10, 64-Bit, Version 21H2, Build 19044
The instinctive need to be the member of a closely-knit group fighting for common ideals may grow so strong that it becomes inessential what these ideals are.
~ Konrad Lorenz (1903-1989)