Did you make the error < 24 hours ago ; if so, then you can cancel without penalty

If it was longer ago than that, then there was a 3rd error in not checking the tickets when received

What was the mistake in booking basic economy? if dates were fixed then why pay extra for a ticket with a high change fee? Many people buy inflexible tickets and quite intentionally so

If it is more than 24 hours ago, it is something to chalk to experiemce and just see what the cheapest fare an airline is offering for that journey ; maybe a disruption will occur that will impact the non refundable journey and entitle a refund