I don’t think I’m the only one out there who thinks that Windows 8.1 is crap, do I? In my opinion, Windows 8.1 is a totally confusing and misconceived operating system, but I think that also Microsoft has the same opinion as I do, since they rolled back a lot of things in the new Windows 10. Anyway, my shiny Lenovo M30-70 shipped with a licensed copy of Windows 8.1. Before getting rid of it, and installing whatever else operating system you wish, it’s a good practice to create a recovery disk on an usb pendrive. Since you paied for the OS, and you could need that license in the future (ex. to reinstall it before selling it, to upgrade to Windows 10 for free…), it’s -warmly- recommended that you backup it. Do it as soon as possible. In my case, Windows 8.1 run for like 20 minutes before I started the procedure, that you can find here. Be aware that you’ll need a pendrive with a size of at least 16 gb, and the recovery disk will take approximately 12 Gb of it. It could be a good idea to backup D: as well, since it contains all the drivers in one place, as I wrote here.
As alternative, you could also avoid to create / backup anything, and stick with the hidden recovery partition available on the computer, but I honestly think is a waste of space to keep that there. I definitely suggest to create a recovery disk on a pendrive (they are very cheap nowadays), backup D:, and then get rid of all the five (!) partitions available on the machine, and create 1-2 partition(s) from scratch according to your needs. In my case, I booted the computer from a Windows 7 pendrive, I deleted the pre-existing partitions and I created two new partitions (100 gb for operating system and programs, and ca. 350 gb for the data). I think that this partition scheme is much better than the default one ;-)