If you have an H-1B visa, or similar, you can study part-time without changing your status.
If you want to become an F-1, full time student, you can apply for a “change of status” and pursue your studies without having to leave the United States. This means you get to stay under a different category.
In some instances, you may not be able to apply for a change of status. For example, if you’ve been admitted to the U.S under a K (fiance) or a C (transit) visa, you are not eligible to apply. It might still be more beneficial for you to leave the United States and apply in an embassy.
For more information about changing your status, click here.