So, before the ep. 6 maintenance I was able to grab the 15* gunblade and watched a video saying that you could change the weapon into another weapon of youe choice by using the batch of 999 items that comes with it.

My question now is where/how do I switch it to a Sword? I the video I mentioned did not say where/how, just says you can do it, I also searched around google, reddit and yt but no luck. Could anyone help, please?

Go to Zig and click "Change Weapon Category".

That's it, now you just browse to what you want to change.