I managed to modify one error message with the help of a plugin named WooCommerce No Shipping Message
Now, I want to modify top error message of my checkout page from this.
No shipping method has been selected. Please double check your
address, or contact us if you need any help.
Not available for shipping to the selected destination. Helpline (+61) 02-3345-5555
Please someone help me to do that, i am not able to find any solution for this. Thank you!