I only ever use netinstall on fresh installs of devices without router os.
If you want to upgrade a unit, grab the latest version for your unit here: http://www.mikrotik.com/download.html
I usually get the combined package for ease of use. Then drag that sucker from your desktop to the mikrotiks root folder via the files menu in winbox.
reboot the mikrotik from system.
upon reboot, provided the package is in the root folder and it is the right package, it will update for you.
Before you update i would suggest exporting all your settings first. Ive had some odd things change when ive done seemingly trivial updates.