Can I return a Mac mini I've bought months ago?

Sorry, but you can only return your iMac within 14 days:

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Standard Return Policy

There are a few important things to keep in mind when returning a product you purchased online from Apple:

• You have 14 calendar days to return an item from the date you received it.
• Only items that have been purchased directly from Apple, either online or at an Apple Retail Store, can be returned to Apple. Apple products purchased through other retailers must be returned in accordance with their respective returns and refunds policy.
• Please ensure that the item you're returning is repackaged with all the cords, adapters and documentation that were included when you received it.

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copied from: Returns & Refunds - Shopping Help - Apple

Why do you feel the need to return your Mac mini?

If you are having some sort of trouble, perhaps if you tell us about it in detail then someone here may provide guidance to resolve the issue.

So that we can help you best, please tell us which Mac, model and year you’re having trouble with, and what macOS version you’re running. Find that info by clicking the  (Apple) menu at the top-left of your screen and then select About This Mac.