There's a link at the top that says "Classic Mail". It's in the line of "Hello [Name] // Sign Out // Preferences // Help // Classic Mail"
If you really can't find it, go to »signin.optimum.net/Webma ··· erChoice
when you're logged in and it will give you the option to switch (choose "no thanks" for the old layout). You can use that to switch to the new layout, too, if you have not had the option.