If this is the one you are referring to...
http://www.gsmarena.com/samsung_galaxy_note_10_1_n8010-4670.php ... then it isn't updatable to "Marshmallow".
You could consider rooting the device, but that can be tricky and dangerous. So much so that it can actually brick your device. Personally I am not a fan of it, but should you wish info on it you can post in the "Rooting, Jailbreaking & Unlocking" Forum.
http://www.tomsguide.com/forum/forum-117.html Yes I know it is under the "Cell Phones" section, however since they are all using Android, the same info should apply.
Now if you are only wanting to gain more room on the device, I would suggest moving things like your photos, music, videos, etc., to the SD card. They take up a lot of...