Hi Juha. As explained in answers below, go to the Edit tab, scroll down to the birth date, correct it, scroll to the bottom of the page, enter a reason for the change, and click the button
FULL SAVE: COMMIT CHANGES
The acceptable date formats are described in detail at
https://www.wikitree.com/wiki/Help:Date_Fields#Input_Formats
(Note that MM-DD-YYYY is an American format, not used in the rest of the world. It can therefore give an unexpected result if you are not American. For example 01-02-1900 gives 2 January, not 1 February. It is safer to use the format "1 Feb 1900".)