Your first step is to contact your former employer or the administrator of the plan. You may want to start with the Human Resources department unless there is a dedicated number or office for retirement plan questions.
If you haven't already begun taking distributions from your plan, you will want to make sure that you start no later than when you turn 70 1/2 years old. (Otherwise, you may be subject to an IRS penalty for not taking your minimum required distributions which are based on your age and life expectancy as determined by an actuarial chart used by the IRS).
If you're looking to withdraw your funds and transfer them somewhere else (say, to an Individual Retirement Account), the same process will apply.
You will then be provided with the right paperwork from the 401(k) administrator to do what you are trying to do.