Run the command below that's appropriate for your operating system, then compare the last few characters of the long string it prints out with the value in the sha256sum.txt file.

For Windows, in a command prompt, run:
certutil -hashfile NameOfISOfile SHA256

For Mac, in a Terminal, run:
openssl sha -sha256 NameOfISOfile

For Linux, run:
sha256sum NameOfISOfile