What exercises can I do to gain flexibility after an ACL tear?

You can check the exercises in from these links and choose which one fits you better: https://exercisesforinjuries.com/4-knee-strengthening-exercises-you-can-do-after-an-injury/

https://exercisesforinjuries.com/safe-and-effective-knee-strengthening-exercises-after-acl-surgery/

For a more comprehensive approach on how to regain flexibility with your knee, you can check this program: http://www.kneeinjurysolution.com/