Social sharing uses two kinds of content: the labels attendees see in HeySummit, and the event details or image that a social network uses when it builds a shared post. Update the part that matches the result you want.
Change the sharing labels in HeySummit
Go to Event Settings > General > Content Translations.
Open Filter records, choose Social under Category, and select Filter.
Find the phrase you want to change, enter the replacement wording, and select Save for that phrase.
Change the event details and sharing image
Go to Event Settings > Branding and Site Settings > Branding & Content. Use SEO and Social Content for your event description, strapline, and social hashtag. Use Additional graphics for the Event sharing image and the Global Speaker/Talk Promo Image. See Branding & Content Information for the full field guide.
Check the final share
Preview the public event page while signed out, then test each sharing option you plan to use. Social networks can cache titles, descriptions, and images, and some networks let the person sharing edit the post text before publishing. Content Translations changes HeySummit interface wording; it does not force a social network to publish fixed post copy.