There are two scenarios:

This means that the lessons or exercise was created by you, or created by a colleague who has granted you editing rights.

To view whether you have editing rights, go to the lesson or exercise and click 'edit'.
If you have editing rights, this is indicated by the unlocked icon in the editor menu and the pen icon in the exercise tile, see the images below.

In this case, you can just change the item as you like.

Do remember that other teachers might use the exercise or lesson as well, so pay attention that in that case your changes will be live for students immediately!

This means the lesson or exercise was created by someone else. You can view it, but you cannot directly change it. 

To view whether you have editing rights, go to the lesson or exercise and click 'edit'.
If you do not have editing rights, this is indicated by the lock icon in the editor menu. 

In this case, you can still copy the lesson or exercise. You then have your own version that you can change as you wish.

To copy a lesson, you must be in editing mode. If you are not, click 'edit'.
Then copy the item by clicking on the 'copy' button all the way to the right in the editor menu.

You will get a pop-up screen with the following message:

Don't check the box in front of 'Force copies of exercises", unless you're sure you want to edit all of the exercises.
Click on copy subject. A copy is then created and placed in your own repository. You have editing rights on this item, so you can now change it as you like. 

You can now make changes, such as: 

You now have your own adapted version of the original lesson, which you can add to your course. Learn how to add the new lesson to your course here.

You can also copy exercises to which you do not have editing rights.
You are inside a lesson to which you have editing rights. But this lesson has an exercise linked to it to which you do NOT have editing rights.
The eye icon on the selected exercise shows that we are allowed to see the exercise, but not edit it. 

However, you still see the copy button on the top right. If you click on this, two options are presented: 

'Replace this exercise in this subject' copies the exercise, provides you with edit rights for that lesson, removes the original exercise and replaces with the new version you have created. 

'Create a new exercise in this subject' copies the entire exercise, provides you with edit rights for that exercise and adds the new exercise, while keeping the original exercise.

'Create a new exercise outside this subject' creates a new exercise and places this in your repository.