So if you're from the UK and own the Honor 6 Plus, Honor 6, or Honor 4X, you can now sign up to become a beta tester for the Marshmallow update for your device. This follows a similar campaign that was outed for the Honor 7 last month. To try your luck at getting selected for this, you have to fill out the form here.
You need to supply your handset's IMEI number (dial *#06# to get it), as well as its current firmware version. This has to be a specific one noted in the document linked above, otherwise you won't be able to participate.
Of course you should keep in mind that such beta builds can be filled with bugs, and make sure you back up your data before you install.