Welcome to the Serenity releases hub, here, you can:
Serenity is a paid client available for $49.99 USD, it uses startup bytecode injection to transform the game — meaning you can use the latest version of OptiFine or even LiteLoader mods.
It also has the benefit of added performance versus other traditional MCP-based clients. They have to decompile, edit, recompile, and reobfuscate the game, whereas Serenity makes slight modifications to the already obfuscated game.
The Serenity click GUI is extremely easy to use and allows for an incredible amount of control over the client. Most of the modules in Serenity have a wide range of options that can all be changed via the Serenity click GUI.
The panels in the click GUI can be opened by right clicking the title. An ellipsis next to a module denotes that there are configurable values available. Simply right click the module to configure these options.