If you've made an error, or just generally need to update or add information to an existing Lead, simply search for the Lead's name and click on it to access the Lead detail page.
Once you're on the Lead detail page, click the small pen to the right of the Lead 'Personal Details' section.
Once completed, click 'Save' and the information will be immediately updated on the Lead detail page.
If the field you need to update is not part of the Personal Details, select the three action dots and select "See full details". Search for the field here and select edit to update