How to switch between new Cycles render engine and classic render engines

From version 12.5a6 there is an option to switch between the new Cycles (GPU) render engine which uses AI and the classic (CPU) render engine.

1. Click Additional modules on the menu bar.
    
2. Select Render Engine.
    
3. Click the option required: either Use new photo render (AI- and GPU-powered) or Use classic photo render (CPU-powered).