How do you upgarde past 5 on a weapon?

The ember for that is in a chest right next to the butcher in the sewers the key for the door is the one you get after killing capra demon.

Once you get an ember you have to take it to a blacksmith. He will go past +5. I think you need a large or dark ember if you want to upgrade a weapon using a bosses soul but I'm not entirely sure.

The blacksmith can go up to +15. +5 is starting out.

When you find a Large Ember, which is found in the Depths, next to a Butcher. Andre can reinforce up to +10.

When you find the Very Large Ember, which is located in the Demon Ruins, guarded by around 8 Taurus Demons, and in the middle of a pool of lava. (Unless you have the charred ring, you will die getting it.) Afterwards, you take it to Andre as well, and he will reinforce +15.

^^
What they said. I just want to correct Peavy_FTW. You get the very large ember in New Londo Ruins after you break the floodgates and go to the lower area of it. The ember that you are talking about in Demon ruins is the Chaos flame ember.