The system does its best to work out a player's starting level based on their first match and then, afterwards, may adjust it if it doesn't look right. In most cases, you can leave the system to do its thing but that can take time and that's no good if players are in the wrong place in the rankings and that's causing trouble.
A player's starting level is included as part of their details and you can edit it on the same page. See above section on how to edit a player's details. You may have to scroll down to find the starting level field. Also note that you can change it but you can also set it 'manually' such that the system won't come back and change it again.
If the player has played a few matches then the starting level is less significant to their current level. This is only really useful to do if the player is new to the system and their first match wasn't representative.