Did you publish the draft?
Changes you make in a draft stay in the draft until you publish it. Open the event, choose the draft in the top bar, and choose Publish. See Drafts and publishing.
If you keep more than one draft, check you published the one you changed: choose each in the top bar to see what's in it.
Is the Publish button missing?
Publish only shows with a draft chosen in the top bar. If it says Not ready to publish, the draft is over one of the event's limits: see I can't publish my draft.
Has the app caught up?
Once the event is published, attendees get the new version in the app. If someone still sees the old one, ask them to close the app and open it again while they're online.
Dates
An event's dates aren't in a draft. They change straight away when you use Change dates on the Overview. See Change your event's dates.