Great question! I’m afraid that the correct answer has to be a bit vague. We’re not actually a web hosting service, but many people do host websites on our platform. This means that FTP is not implied, rather it is something that can be built by the user if they choose to do so, and there is no single correct way for them to build it. In turn, there is no single correct way to change an FTP user/pass.
If you have installed an environment that standardizes this and makes it easier, like a hosting control panel such as cPanel, Plesk, or VestaCP, then you may want to refer to their documentation individually. If you have installed/configured a certain type of FTP server using a certain type of authentication, you can share those details with us here and we may be able to advise.
If you’re not sure about any of this and just looking for the shortest path to get in to the files on the server, you can reset the password for the system user “root” which exists on all servers. We have a quick guide that may help with that: