In all honesty i don't know how to help with the computer tests, i guess you either have it or you don't. I guess what might help is downloading some type of ATC game or similar to help with quick reactions or something.
With the 2nd stage interview tests im not sure because i think they have increased the amount of interviews but the HR interview if fairly straight forward and the technical interview are probably more scenario based questions which can be answered without being in Air Traffic just use some logical reasoning and think about what you say. Other than that, good luck
regards