I cannot update to iOS12 from iOS9.3.3

I cannot update my iPhone to iOS12 from iOS9.3.3.

•Devise: iPhone 5S

Mac Book Pro: OS X EI Captain(10.11.6)

•itunes already updated new version.

•iPhone storage availavility:3.4GB

•iOS12 already downloaded from itunes but when I clicking the update button a message came up it said connecting iPhone SW update center...then shotly after message disappear and it just nothing happen.

Does anyone know the workaround?

Thanks.