Simple, just download the version you want from https://mikrotik.com/download
(make sure correct version).
- drag and drop to file
- system package click downgrade (winbox)
- sustem routerboard upgrate fimware.
According what you wrote on your post, maybe you кnow maybe don't, but i want to tell you MikroTik suggesting to use latest version,but up to you. Your router your business