To turn off notifications for a post simply go to that post and you will see a little down arrow in the upper right hand corner of the post across from the persons name who posted the post. Click that down arrow and a menu will appear. Just choose Turn Off Notifications and that will stop the notifications from coming to you for this post. To turn them back on just repeat the steps and do the reverse.
To change the notifications you receive from the whole group just go to the top of the group page and click on Notifications. A menu will appear and you can choose All Posts: You’ll get notifications for all new posts in the group. Highlights: You‘ll get notifications for top posts and friends’ posts. A top post may be the most liked post or the post that got the most comments from the past few days. Friends’ Posts: You’ll get notifications whenever your friends post.
Off: You won’t get notifications about new posts in the group. You can change these anytime you want.