just go to the shop, click an item and go to "Ranks", there you´ll see what will change
you can improve anything you want: armors, weapons, even bolts and arrows
you can upgrade your weapons anytime to +1, +2 or +3, if you spend 3 points at once or improve the item everytime you get a heirloom point
but as Macropus already mentioned, try to gather up 2 loompoints and then just browse the marketplace, +3 items aren´t that expensive anymore as they used to be
and if you can´t find the item you desire, you can then still upgrade it to +2, which is also pretty good already (though it´s just a little advantage, the best way to get better is pure training! Eu3 for duels and eu2 for fast fights, cause you´ll die a lot at the beginning. Also, you don´t have to wait that long to respawn on EU2 as in EU1, where you always have to wait until the round ends)