I am sorry I wasn't even sure where I should post this, so I just guessed!

My husband bought a round trip ticket and we are needing to get the date changed for his flight home because he will not be able to make it on the orig. date. I do not even know where to begin this process. Who do I call? Where would I find this info? I need help!!! I need help SOON! Any help will be great. Thanks in advance!