Can I still upgrade my iPhone 5s to iOS 9.3.5?

Hi all,

Just wondering if my old iPhone 5s running iOS 8.3 could be upgraded to a non-current version of iOS, e.g 9.3.5.

This is the phone I use for work-outs and running. Not only I do not need any of the new features of ios 11, the only reason to upgrade is one single app that requires iOS 9.3 to connect to dropbox.

If not possible, I'd keep it as it is rather than risking and ending up with a slowish phone.

Any help will be greatly appreciated.

Thanks in advance.

CR