If your booking is eligible, fill out the form at the link below. Bookings done under casino offers or some of the other "sales" will generally not be granted a price reduction approval.
https://www.carnival.com/core/requestforms/priceprotection.aspx
If for some reason the mods remove the link to the Carnival form, just do a google search for "Carnival price protection".
Consider yourself fortunate if they give you credit/refund for the difference in price. Prices dropping is mostly a thing of the past. A lot of us on here miss the days where booking 1+ years out almost guaranteed a price drop before sailing date.