For best of 1, when can you sideboard?

By default, you can't sideboard at all in a standard Best-of-1 — sideboarding is only allowed between games of a match (Rule 403.4), and Rule 403.5 forbids using sideboard cards in the first game, which is the only game in a BO1.

The one exception: the head judge may opt into the altered BO1 setup process, which adds a single sideboard opportunity before game 1 begins (Rule 406.1.g.5). Unless the event announced that change, your main deck stays locked for the match.