Go back into input gestures and go back down to screen curtain where you set it in the first place, and remove it.

Since you can't use your down arrow, try:

1. Hit the reset button in that dialog to reset all your custom gestures, or

2. Use NVDA+f2 before hitting the down arrow to pass it through.

Once you're on the gesture you want to remove the key from, expand it and go down to the key inside it. Hit remove.

Now, to add the correct one, go back to the gesture and hit add.

It'll ask you what gesture you want. Press it.

Then from the menu that comes up, pick what keyboard layout you want. All layouts is a good choice. Note that if you enter the wrong key, hitting escape in the menu will still add the key, so hit Remove and try again in that case.