Are Assassin's Creed: Valhalla runes upgradable?

I've collected a large amount of runes. I've equipped as much as I can onto my equipment and there still a lot that I know I will probably never use because they're either too weak or I just have too many. It would be nice if I could upgrade these to get some use out of them.

Can I upgrade these runes?

Otherwise, I'll probably just sell them off.

No. As far as I can tell, you just obtain new, higher powered, runes as you progress. There's nothing I've seen in the game or web sites that indicate you can upgrade runes.

When you want to sell your low powered runes, you should be able to.