In open leagues, you will have 29 players on your team (23 drafted, 6 minor leaguers). You may only have 25 players at the NHL level at a given time. So if a player becomes injured or is not performing the way you would like, you can send him down to the minor leagues and call another player up in his place.
To do this, go to your GM's Office. You will see all of your players at both the NHL and minor league levels. From here, you can send players down or call them up.
There are two restrictions. You may not have more than 25 players at the NHL level. There is also a 24-hour waiting period after a player is sent to, or called up from, the minors. This means you must wait at least 24 hours before you can move him between levels again.