Good news, it is easier to make something smaller than it is to make it bigger!
If you want to avoid sewing you can look for a local tailor to help take it in for you. This does cost a fee but is good if you don’t know how to sew and aren’t interested in learning.
If you want to do it yourself there are many tutorials for altering/tailoring a garment but they are usually specific to where on a garment you need to make the change. As a good introduction, check out this video: The Beginner’s Guide to Sewing Alterations (Episode 17): Taking In a Side Seam.
If you don’t want to get it tailored and if attempting to take it in yourself isn’t your thing, you can make minor adjustments with safety pins! Just be careful to not put too much stress on one pin, as it might pop while you’re wearing the garment.
Edit: This advice is based on a garment that has minor fitting issues. Since aliexpress and some overseas sites will ask for measurements and provide a cookie-cutter garment that /should/ fit you this is a common issue and comes with the low pricetag on a lot of these costumes. That said, if there is something very wrong with the garment or the alterations it requires are excessive, you may want to pursue a refund instead. (Keep in mind that you won’t be able to get a refund after changes are made to the costume.) Shoutout to delmalani for bringing up returns.