You can change a combatant's initiative at anytime during or before the combat encounter starts.
To do that, look for the D20 icon on the left of the combatant's HP bar on the Combat Tracker. The number on the right is the initiative input, and you can update it just like any other text field. Changing a creature's initiative mid-combat won't affect the currently active combatant, but it might affect which creature goes next.
You can also re-roll initiative for all adventurers and monsters (depending on your Campaign Settings) by clicking the D20 button at the top of the Combat Tracker.