Your one big event of the year is not called "flagship event" by anybody who comes to it. Type your own word once and every heading, menu and page reads it - these screens, the row in the menu, and the event's own page on your website.
Open Site, then Settings.
The section is named after your event already - so it reads "Summit" if that is what you call it, and "Flagship Event" until you say otherwise.
Type your word in "What you call it".
Summit, Conference, Gathering, Family Camp - whatever your people call it. Leave it blank and everything reads "Flagship Event", which is nobody's name for anything.
Fill in "More than one of them" only if adding an "s" would be wrong.
Blank adds the "s" for you, which is right for Summits, Conferences and Gatherings. Type it yourself for something like "Youth Rallies".
Press Save.
It takes effect at once - reload a screen and the headings have changed. Nothing needs rebuilding.
The WEB ADDRESS is a different thing and is not on this screen. /summit, /conference - that is built into the site, so ask us to change it; it takes effect the next time your site is built, and changing it breaks any link already shared.
This renames it everywhere at once, including on your public page. It does not rename anything already sent - an email that said Summit still says Summit.