Open Gmail on a computer
In the search bar, enter: `in:inbox`
Click the checkbox at the top-left of the message list
Click `Select all conversations that match this search`
Click `Delete`
Repeat the process for other categories by searching:
`in:sent`
`in:spam`
`in:trash`
`in:promotions`
`in:updates`
`in:forums`
`in:all`
To permanently remove items from Trash:
Open the Trash folder
Click `Empty Trash now`