how can i delete my previously stored emails on i cloud?

Go to icloud.com, sign into your iCloud account, open Mail, click a folder on the left (such as your inbox), select the mail you want to selete then click on the trash can icon at the top. You can select a range by clicking on the first message, then Shift-clicking on the last. Repeat this for each of your mail folders except Trash. When finished, click on the actions (gear-shaped) icon on the upper right and select Empty Trash.