Not readily available until 29th July, but you have a year to "upgrade" for free. My advice is wait and see if Windows 10 is really any better than 8.1. I use open CPN on Windows 8.1 and it works well, but a lot of software has not even been tested on 10. Don't be a pioneer, remember it was pioneers that got killed by Indians!
Too late now, but you could have signed up for Windows Insider Preview, and have been running 10 in it's development stages. I've been doing so since October, and have been very impressed, with the later versions being very good - and much better than Win 8.1 on the same device. There are also far fewer app compatibility issues in 10 generally because of the architectural changes under the hood.
PS. If you have 7 or 8, 8.1 make sure (just for economic reasons rather than anything else) that you have the free upgrade icon in the task bar - which it will be if you have applied your updates regularly