Bought wrong gift card by mistake

There is currently no way to do this however it has been known for the opposite to be done. You could contact Apple Support and hopefully you'll have some luck, but there is no official common way to change the gift card type.

This thread might also help you Use Apple Store Gift Card to Buy iTunes G… - Apple Community

Unfortunately, you won't be able to. I was mistakenly gifted $200 in app store cards instead of apple store which i had wanted to buy my new computer. Over the past 2 months, I have spent over 7 hours total over 3 phone calls along with numerous emails trying to get a resolution to this same problem. I was misinformed repeatedly and finally my "case" went to an escalation board (after talking to multiple 'senior advisors'). They turned it down and gave me 3 free movie rentals for my troubles but basically said too bad, so sad. Worst customer service experience ever. The ironic thing is, I too was going to use these to purchase apple products. So apple was going to get the money either way. But instead of having good customer service, they decide to scam their customers by having two different cards in the first place that aren't interchangeable. Horrible, horrible experience. Sorry to be the bearer of bad news.

I had the exact same problem. $200 App Store card instead of Apple Store card. Why does there have to be a difference?!?!?! Same company and all but 2 letters are the same and you don’t think people will mess it up?!?!?! I redeemed it not realizing it was an App Store card. They said once it was redeemed they couldn’t refund it.

go here - iTunes - Official Apple Support

This is exactly what happened to me. I wanted to purchase a $75 Apple Store gift card using a Visa gift card I had to make a purchase on the Apple store. Bought an Itunes one by accident. Already spent over 3 hours on the phone trying to get this resolved - I've been cold transferred, disconnected, etc, multiple times with no resolution. Was the WORST customer service experience I've ever been through. Just like Sara said, I was still going to use the $75 to purchase an Apple product - it's pathetic.

See the article https://support.apple.com/en-us/HT204199

Text that says that the card can only be used in the Apple Online Store or Retail Store, or that you can't redeem the card in the iTunes Store.