Assuming your current version is 4.10 and want to upgrade to 4.11 and if failed return to 4.10
So try this:
put both "routeros-mipsbe-4.10.npk" and "routeros-mipsbe-4.11.npk" into your RB433AH
then paste following code in a terminal or ssh screen:
add name=downgrade policy=\
\n:log warning \"### Downgrading Router ###\";\r\
\n/system package downgrade"
add comment="" disabled=no interval=0s name=downgrade on-event=\
"/system script run downgrade" policy=\
then reboot your router. it will upgraded to 4.11 and after that you have 600 seconds to login and in the
System -> Scripts -> Jobs stop the active downgrade script by selecting and pressing minus button
If you can't do this it will automatically downgrade to 4.10 after 600 seconds. also you can change this delay inside script.