So your stud running back just had a season-ending injury and you watched your fantasy season collapse before your eyes. Now you've regained your composure, you're here and you're wondering how you can move him to an IR slot and carry on valiantly. First off, we feel your pain. More than likely the system hasn't updated their official injury status to IR yet (as marked with an asterisk and an "IR" tag next to the player's name) so until that happens he's stuck on your active roster.
If the tag already appears then you're in luck! Just click on the "Manage IR" link at the top of your team page and follow the instructions from there. To move a player back from IR to your active roster do the exact same thing - click on the "Manage IR" link on your team page and then activate the player. It should be noted that the link will only appear in leagues which use IR slot(s), so if you don't see it just complain to your commish until they change the rules next season to include it (if you're in an LM League)!