The mailbox usage tab shows the last login and how much of the mailbox quota has been used, but you can't use it to adjust the mailbox quota for the user in this tab, or in fact anywhere in the Exchange admin center. If you do want to edit the mailbox quota, you'll have to use PowerShell.
After connecting to the Exchange Management Shell, you can use this command to adjust the quotas at which the user receives a warning, at which the user can no longer send mail, and at which the user can no longer receive mail:
Set-Mailbox -Identity "Alara Rogers" -IssueWarningQuota 94.5gb -ProhibitSendQuota 94.75gb -ProhibitSendReceiveQuota 95gb -UseDatabaseQuotaDefaults $false
The Identity parameter can be set to the user's display ...
