Relax. Don't worry about it. The paper is 1 1/2 hours long and include IQ tests and personality assessments. You won't find any books on it because there is none. There are no past year papers as well.
There is however, a teacher's blog which gives some examples of the UPSR Aptitude Test in case you are interested. You can take a look at it to have an idea but there is no need to get your child to "study" it. This will only stress them up further.
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