Just after 5 days of officially announcing Android 6.0 Marshmallow, Google today released the Factory images as well. If you are using Nexus 5 and 6 Smartphones or Nexus 7 (2013) and 9 tablets, you can now taste the new Marshmallow. Simply download the respective firmware from the link provided below and follow the steps provided on the same page for flashing the ROMs manually.
Remember you must have little experience of flashing manually because if something goes wrong, you may brick the device, however that can also be fixed. So, if you want you can wait for the OTA updates as well. Google will not take too long to roll out the same.