I want to cancel the purchase
Buying on approval means that you will get your money back if you regret your purchase and return what you have bought within a certain time. At the store, ask if they allow right to exchange or buying on approval, and make sure that it is recorded on the receipt.
When you purchase a product from a distance, you have the right to change your mind within a certain period of time. A distance agreement is for example purchasing a product via Internet or mail order. When you are unable to physically view the product you are buying you are given the right to return the item without any questions asked. There are some periods of time to change your mind that you need to pay attention to, and exemptions from court where you cannot change your mind at all.
Read more about time periods and exceptions in the article To file a complaint
Note that you are responsible for paying shipping and handling back to the seller if you do decide to change your mind.
I have changed my mind: