You can increase the bonuses of some of the Hero's Legendary pieces of equipment.

This can be done as follows:

1. Select a piece of Hero's equipment in the Stock tab of the Forge. A special marker is shown on the icons of pieces of equipment that you can upgrade.
2. Make sure you have enough of the components that are going to be consumed. You might need Legendary materials and other pieces of equipment to perform the upgrade.
3. Select how many Helheim Fire items are to be used. The more you use, the higher the chance of a successful upgrade. You can choose not to use any Helheim Fire items and only rely on the basic chance of success. Read more about this in the article entitled "How do I use the Helheim Fire item?"
4. Hit the "Upgrade" button.

If the process is completed successfully, the bonuses of the piece of Hero's equipment will increase (and new ones might even appear). The upgrade level will be shown on the icon of the piece of equipment you upgraded.

If the upgrade process fails, you will lose the components that were consumed and the Helheim Fire items you used (if any). The piece of equipment itself will not change.