At times, airlines may change the time of your flight due to schedule amendments or aircraft changes. Changes can be classed as minor, or significant. The definition between the two differs between each airline, and you can check this directly with your airline, however below is a general guide:
Minor: A minor flight change can be anything between 1 hour and 12 hours depending on the airline. When you purchase a flight, you agree to accept any minor flight delays within their terms and conditions of carriage, and generally an airline will not offer any compensation or a refund for a minor flight change.
Significant: If the delay is longer than what your airline defines as a minor delay, you will generally be offered a refund of your flight (you can then look to book with another airline), or an alternative flight (perhaps from another airport or within a more suitable time). Please remember that any refund would only be for the airline enforcing the change – if your flights are divided between airlines then the second airline are not liable to refund their flight.
Your airline will advise of your options depending on how long the delay is for, and what they are offering as a result.
If you want to know what your airline classes as a minor and significant change, please check on their direct websites.