How to get OAuth2 access token from OAuth1 token and secret with API v2

No problem, that would look like this:

[DropboxClientsManager setupWithAppKey:@"APP_KEY"];

DBTransportClient *transportClient = [[DBTransportClient alloc] initWithAccessToken:nil
                                                                         selectUser:nil
                                                                          baseHosts:nil
                                                                          userAgent:nil
                                                                backgroundSessionId:nil
                                                                      delegateQueue:nil
                                                                             appKey:@"APP_KEY"
                                                                          appSecret:@"APP_SECRET"];
[DropboxClientsManager authorizedClient:[[DropboxClient alloc] initWithTransportClient:transportClient]];

[[DropboxClientsManager.authorizedClient.authRoutes tokenFromOauth1:@"OAUTH1_TOKEN_KEY" oauth1TokenSecret:@"OAUTH1_TOKEN_SECRET"] response:^(DBAUTHTokenFromOAuth1Result *result, DBAUTHTokenFromOAuth1Error *routeError, DBRequestError *error) {
    if (result) {
        NSLog(@"OAuth 2 access token: %@\n", result.oauth2Token);
    } else {
        NSLog(@"%@\n%@\n", routeError, error);
    }
}];