To upgrade a piece of armor, you will first need to craft it. You cannot upgrade your original armor. Once you have crafted a piece of armor, you can upgrade it at Brok's shop.
To do this, go to Brok's shop and select the "Upgrade" tab. Then, select the piece of armor you want to upgrade. You will need to pay Hacksilver and Soft Svartalfheim Steel to upgrade your armor. The cost of upgrading will vary depending on the piece of armor and its level.
Step-by-step:
1. Craft a piece of armor.
2. Go to Brok's shop.
3. Select the "Upgrade" tab.
4. Select the piece of armor you want to upgrade.
5. Pay the required Hacksilver and Soft Svartalfheim Steel.
6. Click "Upgrade".