I will try to make it simple.
1. click on the notifications box at the top right after your name
2. click on your inbox to display the messages
3. at the right side of the message is a small white box, click on each box of the messages you want to delete, a check will appear
4. go to the bottom of your displayed messages, you will see a blue box that says selected messages, click on it
5. there will be several drop down instructions, select delete
6. select proceed to delete the selected messages
I would suggest starting with deleting one message, then move on to selecting more messages. Practice will make perfect
I see Mrs. B is quicker than I am. LOL. Between the both of us you can "git er done".