You don't. Archiving is a feature of gmail. To delete an email in gmail, move it to the deleted items folder. When you switch to another email account that doesn't support archiving, the Archive will change to Delete.
If you are talking specifiaclly about Gmail in your Mail app, you can also go to your Settings app to change that behavior for Gmail.
Go to Settings > Mail, Contacts, Calendars and select your Gmail account. You should get a popup and one of the last lines in that popup should be an option to "Archive "Mail." Set it to OFF and the next time you go into your Mail app, you will have a Trash file instead of Archive for your Gmail account.
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