Help moving “available” $$ from one credit card to another card

I'm a new YNAB user and need help thinking through a credit card payment step. I have a payment coming up soon (credit card A). I was going to send the "available" amount in, which is more than what I have assigned by about $400. However, I am paying down the balance on another card (card B) and I realized it's more beneficial for me use those extra available funds from Card A to pay down Card B. Last week, I sent a payment in to Card B, but now I want to send an extra $400 to help get at the balance & interest. Is there a mechanism or workaround where I can move those extra dollars ($400) from my Card A to my Card B category? Do I change the assigned amount or the available amount? I can't make sense of how to do it. Thanks all!