How to archive mails to USB?

I need to know a method to archive my mails to my USB because I am running out of space. I know it’s an unrelated question but please suggest any possible method.

@NeilGarrett There are many ways to archive or backup your mails to USB such as manual, online and offline tool. I suggest you to search the web for an offline backup tool.

Thanks @DinaTaylor but now I have found the solution to archive my mails and i.e. Mail Backup X.