When your User account for logging in to the Back Office system is created, the administrator of your business account sets the permissions for your account.
Thus, it may be the case that you do not currently have access to the Setup module.
If you require something to be changed in the settings for your business account, you should ask the account administrator to make the changes.
If you should have access to the Setup module and don't, your administrator is able to give you access to this.
The administrator should see Setup > Organization > Users
If you are not sure who your business account administrator is, just send us a message using the Help button on the bottom right hand side of the page when in the Back Office and we will be happy to help.