Hi visa1416,
Unfortunately, you can't go back and change the timing once you've started an exam. The next best thing would be to take the test with a stopwatch or use your computer clock to keep time.
In general, keep in mind that your performance on practice tests will vary depending on the conditions you set for yourself. The more GMAT-like the conditions are, the more accurate of an assessment you'll have. With that said, though, it's important to not place too much emphasis on the actual score you receive because the practice test can only generally approximate your "true" ability level. Instead, view your assessment reports after the test is over to gauge your relative weaknesses and spend more time studying these areas.
I hope this helps. Good luck studying!
Best,
Michael Dinerstein
Manager of Online Marketing
Manhattan GMAT
212-721-7400
800-576-4628