In Rebex FileServer component, you can change the SSH software version string using the following code:
var server = new FileServer();
Rebex.Security.Cryptography.CryptoHelper.SetOption(server.Settings.SshParameters, "SoftwareVersion", customSoftwareVersionString);
However, please be aware that those strings are intended to be used by SSH implementations to determine whether to apply various workaround or features - see RFC 4253:
The 'softwareversion' string is primarily used to trigger compatibility extensions and to indicate the capabilities of an implementation.
This means that changing the string could potentially trigger interoperability issues that would not occur otherwise.