Hello Elisabeth You can open a new discussion when you change topics, and I expect you will have many more questions as there is a lot to learn.
There are two answers to this question, depending on what you want to do.
1) You can move the bones and cartilage components in Spaceclaim to a new knee angle, making sure to maintain contact, then stretch the ligaments with the Move tool by picking half the faces.
2) You can delete the ligaments from CAD and insert nonlinear springs in Mechanical to create the ligament forces in the simulation.
Since you are interested in studying the impact of a knee against a surface, I suggest you get some better geometry that includes the Patella, which is often damaged in an impact. Though it is not labeled in the figure below, there are more than two ligaments in the knee, such as the ACL which is often torn.
Regards Peter