First off, you gotta remove all the covers.
Second, in my country, I use hot thinner or alcohol 99% to clean the board. You can either brush it for a long time or use an ultrasonic cleaner.
Then, dry the board with hot air. After that, apply flux and use the right amount of heat to melt and spread it under the IC, re-balling the solder balls.
Be careful of high heat on the processor or flash
Finally, before you power up the board, you need to check the traces and make sure there's no short circuit.