How can I turn hide my email off

Hide My Email is a service which is part of iCloud+ and can't be turned off. However, to stop using an alias email address you have created with Hide My Email follow Apple's instructions in:

Create and manage Hide My Email addresses in Settings on iPhone

or

Stop using, reactivate, or delete a Hide My Email address on iCloud.com

Apple also provides a You Tube Video for instruction:

here