The simplest way is the 
Manage page — find Race calendar and add, edit, or remove races there. Each race takes a date, distance, name, priority, surface, course difficulty, and an optional target time. Saves are instant and your dashboard reprocesses within a few minutes.
Changes land on your dashboard within about 10 minutes of your next Strava upload, or on the nightly refresh if you don’t run again before then.
Full explanation: Manage Page in the guide.