How do I set it in D&D Beyond? There is no box for me to check. The previous weapon is already unchecked.

What is the new weapon? Are you proficient with it?

Are you using the App or the website?

You can only do it on the webpage. Doesn’t let you on the App. 

If it is two-handed you can only set it as a hex weapon if it's also a pact weapon. 

Is the new weapon two-handed?

It's the staff of the archmagi, which I am proficient with. I am using the website.

Ahh. That registers in the system as a “staff” not as a “weapon.” You will need to create a homebrewed copy of the SotAM and change its item type from staff to weapon->quarterstaff. Then add that version to your character sheet instead.

Okay, my DM is going to look at it. I assume he will make the changes.

Don’t forget to remove the OG version from your character and add the altered version.

okay