https://ndcdyn.interactivebrokers.com/sso/Login?action=ACCOUNT_TYPE

As selling short involves borrowing (of stock rather than cash) and poses unlimited risk (as there is no limit as to how high the stock price may increase when the position is eventually closed), a “Margin” type account is a prerequisite to entering orders to sell short.

If you manage a “Cash” type account which doesn’t allow such borrowing and wish to request an upgrade to a “Margin” type, please log in to Client Portal/Account Management and select the Settings and then Account Settings menu options. You will then be presented with a screen through which you may request upgrade of your Account Type.

Note that account upgrade requests are subject to review and approved by a Compliance Officer prior to the upgrade taking place.

Find more information on how to change Account Type in Client Portal Users’ Guide.