You'll be able to request a refund by reaching out to the app developer directly. If you installed the app from the Shopify App Store you'll find the developer's contact details directly on the app's listing. Here is an example:
If you run into difficulty getting in touch with the app developer, then Shopify Support can request a refund for you. You're able to contact Shopify Support using live chat, email, or callback via the Shopify Help Center. This video clip shows you how to access the chat and email option. If you would prefer to speak with someone over the phone, please log in to your store when prompted to view the callback option. It's good to note that callbacks are only available when capacity allows, so for the fastest support, I do recommend selecting the chat option.
If you installed the app from a third-party website (not Shopify), then I recommend reaching out to the website you made the purchase from to discuss refunds.
To learn more about how to access support for your apps check out this handy help page from the Shopify Help Center.
Hope this helps. If you'd like any recommendations for an app I'd be happy to assist. Please let me know what you'd like to achieve and I can look to see which apps will be best suited.