Since the official website now promotes TallyPrime, you must navigate to the archives to find this specific legacy version.
Download TallyPrime Support Files, Lastest to Previous Version
: Choose Tally.ERP 9 Release 6.6.3 from the dropdown menu and click Install .
: Go to the Tally Solutions Download Support Files page.
: Scroll down to the footer or look for the section titled "Install a Previous Version" .
Tally.ERP 9 Release 6.6.3: Installation and Activation Guide
To ensure the software runs smoothly on your Windows PC, verify these minimum specifications:
: Trusted third-party mirrors like FileHorse or FileHippo also host archived setup files for this specific version. System Requirements for Release 6.6.3

