In a single day event or tournament, Generate Matches rebuilds every round and clears the existing matches, so it disappears from the Admin menu once any match has a score. To rebuild only the later rounds and keep the scored ones:
Tap Admin > Add/Regenerate Rounds.
Choose Regenerate Rounds.
Pick the round to start from under Regenerate from round, then confirm.
In a league shown By Date, tap Admin > Regenerate This Date and choose From a round instead. Rounds that already have scores must be cleared before they can be regenerated.