Just like allowing students to submit late, go to the Late submissions tab on the assignments page. You can either allow all students to give late feedback or select students individually. 

Note: When you use "Allow all to hand in 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.