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

To change the Base Currency, login to Client Portal/Account Management and select Settings followed by Account Settings from the menu. Scroll down to the selection titled “Base Currency” located in the Configuration panel to see your current election. Click on the gear-shaped icon to change currencies.

Changes to this setting generally take effect the following trading day. Please note that IBIS, EmployeeTrack, Flat Fee Referrer, Simulated Sports Betting, IB India and IB Japan domestic accounts cannot change their base currency.

It is important to note that changing your Base Currency does not convert existing cash balances or change the denomination of a security into any other currency. See: Will changing my Base Currency impact my account balances?