Index: src/wp-admin/update-core.php
===================================================================
--- src/wp-admin/update-core.php	(revision 50503)
+++ src/wp-admin/update-core.php	(working copy)
@@ -55,15 +55,23 @@
 		$current = true;
 	}
 
-	$is_development_version = preg_match( '/alpha|beta|RC/', $version_string );
-
 	$message       = '';
-	$submit        = $is_development_version ? __( 'Update to latest nightly' ) : __( 'Update now' );
 	$form_action   = 'update-core.php?action=do-core-upgrade';
 	$php_version   = phpversion();
 	$mysql_version = $wpdb->db_version();
 	$show_buttons  = true;
 
+	// Nightly build versions have two hyphens and a commit number.
+	if ( preg_match( '/-\w+-\d+/', $update->current ) ) {
+		// Retrieve the major version number.
+		preg_match( '/^\d+.\d+/', $update->current, $update_major );
+		/* translators: %s: WordPress version. */
+		$submit = sprintf( __( 'Update to latest %s nightly' ), $update_major[0] );
+	} else {
+		/* translators: %s: WordPress version. */
+		$submit = sprintf( __( 'Update to version %s' ), $version_string );
+	}
+
 	if ( 'development' === $update->response ) {
 		$message = __( 'You can update to the latest nightly build manually:' );
 	} else {