The "Swap runes" service allows you to swap runes of different items you own (whether equipped or in your backpack) at the blacksmith.

To swap runes drag an item onto the corresponding area at the blacksmith or use the right-click context menu on the item whose rune you would like to swap. A window will open where you can select the target item and rune.

Swap runes

Swapping runes costs 20 mushrooms.