I've decide to install Ubuntu 15.4 along side windows 8.1. What should i watch out for and will it be okay to upgrade to windows 10 with Ubuntu installed?
Back up your data as always, better safe than sorry.
If you already have 8.1 installed, ubuntu will do the heavy lifting creating the boot loader for you so you can select your preferred boot environment every time you turn the unit on.
Download, burn, and boot into your preferred live-disk to verify that your machine will function properly in the distribution of your choice.
Run the install process when you're ready, shrink/reallocate space in the installer, and tell ubuntu where you'd like it to install, enjoy your dual boot.
Oh and yes you'll have no problem updating or doing anything in the windows environment.
Always install windows first and then linux.
If grub (linux bootloader) does not work properly (either crashes or you always boot to windows) use this:
https://help.ubuntu.com/community/Boot-Repair
from the live usb or disk
If you cannot see windows in grub login to linux and run in terminal ¨sudo update-grub¨