my zacian wanted to learn giga impact and I accidentally misclicked. now I really want behemoth blade back because I'm getting into vgc.
If you enter any Pokemon Center and speak to the man with the Indeedee on the left of the building, he will give you the option to let your Pokemon relearn a move. In previous generations this required a Heart Scale, but he will let your Pokemon relearn moves an infinite number of times. It should be a straightforward matter to get Behemoth Blade back once you find him, as all you need to do is find Iron Head on the list and give a Rusted Sword to Zacian to turn Iron Head into Behemoth Blade.
Source: Experience