This page on the IBM support site shows 9.1.0 is supported on FS5300 systems:
Hardware Interoperability Matrix for IBM Storage FlashSystem 5x00 Software Levels
https://www.ibm.com/support/pages/node/5694123
And this page shows the supported upgrade paths, indicating 8.7.0.2 to 9.1.0.0 is supported:
Concurrent Compatibility and Code Cross Reference for IBM Storage Virtualize
https://www.ibm.com/support/pages/node/5692850#_V8.7.0
If you want/need the GUI to download the code directly to the system, you could raise a support ticket to investigate. Alternatively, you could download the code from Fix Central (links on the page above) and then use the GUI to upload that to the machine from your workstation.