If you are looking to assign a different assignment to one (or a few) students on a regular basis you to create a new, separate class for those students.
Create a new class. (how to create a class)
Move the students needing adaptations into this new class. (how to add existing students to a new class)
Click Assign on the assignment that you want for this small group of students in the new class, while assigning a different assignment to the original class.
This is not a realistic solution if you only want to do this once or twice. This is a viable solution if you want a different assignment given to these students on a somewhat regular basis.
It is NOT possible to assign work to ONE student within a class and not the others. If you assign work to a class it will go to ALL students within that class.