When you go to About phone and then tap on System updates at the top.....does it offer an update ? Having the phone fully charged seems sometimes to trigger the offering.....tho' I've got a Motorola......
You may be dependant on your network provider for updates.
I tried the "check for updates" thing. Phone is up to date. Last week I got an update but it still says 5.0, just thought it would be 5.1 or something buy now.
Just had a quick look thro' a Search ......seems that you're up to date.....see here :-
https://www.google.co.uk/search?q=What+version+Android+is+S5&rlz=1CASMAE_enGB648GB648&{google:acceptedSuggestion}oq=What+ve