Some "online" patchers actually run client-side in JavaScript (using emscripten-compiled xdelta). These are truly online but limited by your device’s RAM and CPU. Others upload files to a server – avoid those for sensitive data.
If you want to avoid installing software, these browser-based tools are highly recommended: apply xdelta patch online
True online patching (pure WebAssembly) is improving. As browsers gain better file system access (File System Access API) and increased memory limits, fully local xdelta patching in a web page may become practical for multi-gigabyte files within 2–3 years. If you want to avoid installing software, these
by Marc Robledo: Highly regarded for supporting a wide range of formats (xdelta, BPS, IPS) with a fast, modern interface. kotcrab xdelta-wasm kotcrab xdelta-wasm For large ISOs (4GB+), stick to
For large ISOs (4GB+), stick to portable native apps like DeltaPatcher. However, for retro game ROMs and small updates, the future of patching is browser-based. The next time you need to patch a file on a locked-down PC, open a browser tab instead of opening your installer—just make sure you check your RAM first.