You certainly can!
On the assignment you would like to change, just click the blue link next to "Deadline" or click the edit button on the right hand side.
The options for the deadline are 30, 60, 90, 120, and 180 days from when the training is launched. Note that you are required to set a deadline when creating an assignment so this will already be on any assignments that are scheduled or sent out.
If a training assignment is scheduled to be sent out, you can still adjust the deadline and change the date.
Once training is assigned to your team, it is possible to change the deadline. If you make a change, this will apply to all team members and if extended will convert those who were marked as Incomplete back to Within Deadline.
Please note that Deadlines are based on calendar days and not business days.