A player arrived late
In most tournaments, you can simply switch to the Players tab and add the player. For match play-style tournaments, the player will be included in the next round of play. If your tournament is using four-player groups and you have a three-player group available, you can click the cog icon, then Modify player and there you can add the new player to an existing group.
In elimination bracket tournaments, it is not possible to add players after the tournament has started.
A player left early
When a player leaves early, go to the Players tab and deactive the player. The player will not be included in the next round in a match play-style tournament. If the player was part of an active match, you can click the cog icon, then Modify player and there you can remove the player from an existing match.
In elimination bracket tournaments, it is not possible to remove players after the tournament has started.
Wrong results entered
If the current round has not ended, click the cog icon and then the Delete results button. That will let you re-enter results for the match.
If the round has already been closed out, switch to the Players tab and then the Adjust points or Adjust strikes sub-tab. That will let you manually adjust points or strikes for the effected players. See more under Scoring.
Handling arena malfunctions
At any point, you can pick a new arena for a match by clicking the cog icon and then the Select arena button.
Adding/removing player from existing match
If you need to manually move players between matches, click the cog icon followed by Modify players. That will open up a dialog where you can add, remove or replace players from the match. If you need to switch two players, that requires a two-step process where you have to Modify players for each of the two effected matches in turn.