update (opts) {
    opts = this._defaultAnnounceOpts(opts)
    if (opts.event) delete opts.event
    debug('send `update` %o', opts)
    this._announce(opts)
  }
Send a `update` announce to the trackers.
@param {Object} opts
@param {number=} opts.uploaded
@param {number=} opts.downloaded
@param {number=} opts.numwant
@param {number=} opts.left (if not set, calculated automatically)