There are two ways to delete an ascent:
Navigate to your profile, tap the Ascents List tab (second from the left), and scroll to the climb you want to delete. Tap it, then tap the ellipsis (···) in the top right corner. Tap "Delete," then confirm.
Open your logbook (the lightning bolt icon) and tap the date of the send in question. Scroll down and tap "Edit Logbook." Scroll back up to the climb, tap the ellipsis (···), and delete it.