Last name

I have a question, every time I join a zoom meeting it displays my last name no matter how many times I change it to display only my first name. Even if I check to make sure it just says my first name in display in my profile right before I click the link to join a meeting, it always goes back to show my last name and I have to change it during a meeting after it’s too late and everybody already saw my last name. How can I make sure it won’t display my last name next time I join a meeting???