I believe there are some aspects of psychometric tests which you can not change, however there are many parts that can be improved vastly. Also, different tests can have different results completely. I personally have completely flunked one set of tests and aced another just 2 months later. It really makes no sense to me and confirms my suspicion that the only real way to assess flying ability is to sit you in a sim. I'm certain that how you are feeling on the day must be a huge factor too.
Why don't you invest in a book or two on psychometric / aptitude testing? I'm sure you'll find it useful in understanding what you can and can not change and how to do better next time. If there really is a problem, it's best to know now rather than shelling out thousands of yours (or possibly someone else's) hard-earned cash to find out.
Good luck!