Dear, i have the rb750 geupdated with the 5.2 file, but in will back downgraden to ros 4.9.
I have two quistions, 1 where find i the 4.9 ros?
And how replace his back on rb 750
Pleace help me.

You can download the latest 4.x (4.17) from the official site: http://www.mikrotik.com/download.html

Thanks for the reply, i have done what you told, downloading the file from microtic and drag the file into the file window.
The file beeing upload to the router but if i reboot the router is it still version 5.2.

Look at the logs. What do they say?

log sayd nothing the routerboard is unconnected from network this bgp errors are normal

Right after the reboot the logs will state the reason an upgrade failed. Reboot again, check logs.

Oh yeh, i see the problem.
which allows me to do

You need to issue the downgrade command:
http://wiki.mikrotik.com/wiki/Manual:Sy ... #Downgrade

thanks, is worked with command:
"system package downgrade; /system reboot"
You was a great help, thank you very match