because my wAP ac always crash with 5G wifi, I think the new version is not friendly for wAP, it looks software issues, which may not be fixed in a short time. I want to downgrade to an old version. but who can tell me where to download older versions? I just can't find it. thank you.

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)
- reboot
- 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