Upgrading to Analytics 2.5.6 is supported only from 2.5.5 using SWI files. For details on how to upgrade to Analytics 2.5.5, refer to the Upgrading On-Premises Analytics section in the below deployment guides:
For HyperV - Analytics Deployment Guide - HyperV
For ESXi - Analytics Deployment Guide - ESXi
For Azure - Analytics Deployment Guide - Azure