Notoriety can be reduced by doing one of the below three things:
Take down Connor wanted posters (-1 level).
Bribe town criers (-2 levels).
Bribe printers (-3 levels).
The location of each of the above will be marked on your map whenever you earn the first level of notoriety. In order to lower it, approach a poster, crier or printer and interact with them. You can do it only when you're not chased or attacked by enemies.