You do not need an attorney. However, the I-539 form has five pages of questions. Can you answer them all? For example, are you eligible to extend your status, or must you instead apply to change to another status? If you entered as a B-1 visitor, for example, you might need to change to B-2 if you have no more business meetings planned.
If you have a fairly good understanding of US immigration law as it applies to your status (and, if applicable, the status you wish to switch to), then you can probably fill it out, but if you are uncertain about anything then it might be a good idea to engage a lawyer to advise you or to prepare the application for you.
I would suggest that you print the form, fill it out, and make note of any uncertainties. If they are sufficient that you hesitate to send in the form like that, then you should probably call a lawyer. When you do, you'll have a much clearer idea of what to ask for by virtue of having completed this exercise.