Upgrading an older computer from Windows Vista to Windows 8.1 is a significant step forward in performance and interface, but it is not a direct process. Because Windows Vista and Windows 8.1 are separated by two generations (Windows 7 and Windows 8), there is no official "in-place" upgrade path that keeps your programs intact.
Before starting, ensure your Vista-era hardware can handle the newer operating system. While Windows 8.1 has modest requirements, older machines often need a RAM boost to run smoothly. Windows 8.1 minimum requirements update windows vista to windows 8.1 free download
To successfully update, you must perform a or follow a multi-step "leapfrog" upgrade. Core Upgrade Requirements Upgrading an older computer from Windows Vista to Windows 8