You can allow students to submit work late by going to the Late submissions tab on the assignment page.

To allow for late submission for all students click Allow all to submit late or you can select which students submit late by clicking Allow next to the student's name. 

Note: You can only allow a student to submit late after the deadline has passed.

When you use "Allow all to submit late" and set a deadline, you override any deadline you have set for students you have already allowed to hand-in late. 

Similarly, if you choose "no deadline" when allowing all to hand in late, any deadlines you have set when allowing individual students to hand in late will be removed.