Why update the core
v2rayN is the interface; the actual work is done by the Xray or sing-box core. REALITY, new ciphers and bug fixes all ship with the core, and its version is separate from the interface's.
One-click update
Under Check for updates you'll find "Update Xray Core" and "Update sing-box Core". Clicking downloads from GitHub and swaps it in; restart once to apply. If the download fails it's usually GitHub connectivity — connect to a working node first, then update.
Manual replacement
Download Xray-windows-64.zip from the XTLS/Xray-core Releases, extract xray.exe, and overwrite the same file under bin/xray/ in the v2rayN folder.