For you to leave a group in LinkedIn app:
In your LinkedIn home page you will find your Interests at the top. Move the cursor over your Interest and then select Groups.
Now, find the group you want to leave and click on that name.
Find the Member button at the top right > then move your cursor over that > now, the text would change to Leave.
Click on Leave.
You would be asked to confirm your choice then click Leave in the window.