There is not much information to go on - but this link may help get you started if it is wartime SBS you are talking about (Army Unit)
https://www.gov.uk/get-copy-military...-elses-records
https://www.gov.uk/government/public...-request-forms
If it is more modern you need the Admiralty or Royal Marine equivalent
Note: you may need a photocopy of your relatives Death Certificate to apply.
The process appears to have changed a bit since I last used it pre-Covid - it took a long while to come through given Covid office closure. I am not sure what the waiting time is now.
Hoping this helps
Mike