Open Outlook
Go to the folder containing the emails
Select one email in the group
Hold Shift and click the last email to select a range
Hold Ctrl and click to select non-adjacent emails
Click Delete or press Delete on your keyboard
To delete all emails in a folder, press Ctrl+A and then Delete
To delete emails from a sender, use the search box to filter them first
Select the filtered emails
Click Delete
Empty the Deleted Items folder if needed