i'm running Windows 10 on a laptop, upgraded from 8 to 8.1 then 10, cpu is core i5 4200u, can it handle Ubuntu?
if i installed 15.10, can i upgrade to 16.04 LTS when it comes out?
Yes your i5 can surely handle ubuntu 15.10 and 16.04. I have an i3-4030u running ubuntu 15.10 and its blazing fast. And also you will be able to upgrade from 15.10 to 16.04 when it comes available to public.
yes, ubuntu system requirements recommends 1GHz and 2Gb ram for gui usage, so a core i5 should be no problem at all.