This is very difficult to know and check when you are booking online. There are two classifications for tickets, regarding refund or exchange. There is a refundable ticket or a non-refundable ticket. You can change plans and not pay any fees. Flights on Tuesday, Wednesday or Thursday are typically not cheaper, because of there high price.
Some flights are automatically refundable, because of there high prices.
But, you may want to make sure you are getting a refundable ticket if you are unsure of your plans. Non-refundable These tickets are the ones that are typically discounted. Sometimes the fair classification will give you a cheeper rate, for the current price of the ticket. This fee is an industry standard. If you change your plans and pay this fee, your new ticket will also be non-refundable.