If you're looking to modify your Pokémon save data directly in your browser, the primary tool available is PKMDS for Web. Unlike standard desktop applications, this web app allows you to upload, edit, and download your save files without installing software. Core Guide: Using PKMDS for Web
While desktop apps remain superior for safety, several online tools have gained popularity. Note: The landscape of these sites changes frequently due to Nintendo DMCA takedowns.
Enter the Online Pokémon Save Editor.
PKHeX: The current gold standard for save editing, this desktop application allows users to modify almost every variable of a save file, from Individual Values (IVs) to trainer information.