SharePoint 2013 and 2016 don’t have that old and familiar “sign in as a different user” -option in the site menu, and for the time being, nor does SharePoint Online. However, sometimes it’s very useful functionality to have, so it’s a bit weird Microsoft chose to get rid of the option. This post outlines a method that I’ve found to work quite well!
Solution for logging in as a different user
There’s an url you can use to automatically log the current user out, and prompt for new credentials.