The most efficient method would be to call the investment company offering the plan, their contact information is probably on your statements. If you don't have a statement, contact your employer, they should be able to point you in the right direction.
Just a few points to consider, if you have not already: If you are under age 59 1/2 and still working at the employer where you have been funding the account, you may not be able to take any distributions at all, per IRS regulations. You should, however, be able to take a loan from the account or possibly even a hardship distribution. Again, the company administering the plan will be able to walk you through all this.
You will also want to consider the tax implications of a large withdrawal as all of it will be taxed at ordinary income rates, which could bump you into a higher tax bracket. If this is going to be a possibility, you might want to consider splitting the distribution between this tax year and 2018.
As for closing the account, a total distribution usually closes the account automatically.