Practice websites
You cannot study for a Cognitive Abilities Test but research has shown that the "practice effect" can make a difference in scores for students who take the test more than once. In addition, students can use practice sample tests, books, worksheets, and websites which familarize them with the format of the test questions. Students in grades 3rd-8th have a time limit per section, so understanding the question types beforehand can help them work more quickly and reduce test anxiety.
You can find lots of FREE practice, without having to pay for anything.
Video from Testing Mom- What does the CogAT assessment look like?
https://mercerpublishing.com/cogat
https://mercerpublishing.com/cogat/sample-questions
NGAT practice
NNAT practice