Your question poses an ethical dilemma.
If somebody is looking for ways of "cheating" on a test, I hope nobody helps them.
However, if somebody is simply asking for clarification of what to expect, and how to avoid "false" results or inadventently failing, that‘s fair ball.
One of my friends (a marathon runner) had problems passing the step test when it was first introduced - his heart rate would shoot up too quickly in the first couple of dances, and they wouldn‘t let him go any further.
Out of frustration he went to a civilian fitness clinic where they put him through a more thorough battery of test - they determined that his heart rate did indeed increase quickly ... but then levelled out at quite a reasonable plateau, and went on, and on, and on, and on, and ... (you get the picture - a veritable "Energizer Bunny")
Also - please keep in mind that this and many other tests are intended to be a measure of your fitness. "Tricking" the test isn‘t the aim - you‘re only fooling yourself if you do, and you‘ll fail later on ...
Now, having said all of the above, let‘s talk about any test:
People often get nervous when facing the unknown, and their heart rate can go up ... (i.e. a test could unwittingly becomes a test of nerves, instead of ... whatever).
So, the best advice is to stay calm.
Ultimately, you know how fit you are, and you should be confident that you‘re fit enough to join the Army.
To avoid "false" readings due to unnecessary nervousness, get familiar with the environment you‘ll be tested in (i.e. remove the fear of the unknown).
There‘s no law against practicing, either, as long as it‘s not for the intent of cheating (i.e. if the ONLY cardio exercise you do is the stair climber/master, you‘re only fooling yourself - you‘ll fail once the test criteria changes to running or marching with a full rucksack ...)
And, since you know how to use the Internet well enough to arrive here, you should be able to do your own research by searching for "step test" or "Harvard step test".
Good luck.