Depending on your security permissions, you can edit time entries after submitting them to your manager, etc. or even after they are approved. Approved time entries can also be deleted. You can use the Security screen to allow or restrict that action.
To limit users from editing the time entries of other employees, go to Security> Time Entry> Check 'Limit editing to self only' option. Similarly, make sure you have the 'Allow Delete' permission checked to permit deleting time entries. If you are unable to delete an approved time entry, first un-approve the time entry and then proceed to delete it.