For many competitions, upsolving is available after they're complete. You can go back to problems you did not have time to solve during the contest or do more work on problems you already submitted solutions for. These solutions are not counted in the tournament table or considered by the jury, though they are checked on the testing server. There are no time limits on upsolving.
To return to a contest, find it in the list or use the direct link provided by the organizers. If the contest is accessible for upsolving, its page will display the status “Your participation in the contest is over. You can submit solutions”. Problem are solved the same way they are during regular contests.
Participating in contests virtually
In virtual participation mode, you can solve problems for some already complete contests you did not participate in. This mode is as close as possible to participating in a real contest: all the contest rules apply, including the time limit for the contest duration.
Virtual participants are not displayed in the contest's official tournament table.