On another thread pipkin wrote:

'yeah, just disconered that too, now have max fire and still only 90% success rate ... Slider works - drops to 60% ' 

not all items can be updated, i.e. it seems to be just the fighting gear.

Yes, I knew that bit,I think the text says as much, and that any equipment that can be upgraded will be highlighted.

 Perhaps that is Joe's 'problem', its not clear from the post. 

The items in your equipment section of your forge with arrows against them can be upgraded. They are special gear

The slider appears on the screen where you upgrade the equipment.

So go to your Forge and click on Stock.  Now look for the items with the upward pointing arrow on the top left of the item's picture.  Click on one of those pictures and then on "Upgrade".  Now you see a screen which shows, in the upper half, the legendary items needed to upgrade the equipment to the next level - and which of those items you already have.  If you have all the legendary items required, the slider allows you to regulate the amount of Helheim Fire which you wish to apply.  Applying Helheim Fire improves the chance that the upgrade will succeed.

If you are like me you will always apply the maximum amount.  It is frustrating to fail an upgrade and to see a bunch of legendary items disappear for no gain.

In my own case the equipment which I have crafted which can be upgraded is Rollo's gear and Ubba's set for my hero, Sleipnir's set for Elsa, gear which improves scouting for Wendla and gear which improves Raine for attacking and defending fortresses.  The introduction of upgrades has certainly had an effect on gameplay as heroic quantities of items are needed for the upgrading process.