How do you change from a S corp to a C corp on Turbo Tax Business 2018?

have you file an election to terminate or did one of the automatic termination conditions occur?

AUTOMATIC

A) no longer qualified to be s corp -fail IRC section 1361(b)

1361(b) Small business corporation
(1) In general for purposes of this subchapter, the term “small business corporation” means a domestic corporation which is not an ineligible corporation and which does not—
(A) have more than 100 shareholders,
(B) have as a shareholder a person (other than an estate, a trust described in subsection (c)(2), or an organization described in subsection (c)(6)) who is not an individual,
(C) have a nonresident alien as a shareholder, and
(D) have more than 1 class of stock.
(2) Ineligible corporation definedFor purposes of paragraph (1), the term “ineligible corporation” means any corporation which is—
(A) a financial institution which uses the reserve method of accounting for bad debts described in section 585,
(B) an insurance company subject to tax under subchapter L, or
(C) a DISC or former DISC.
(3) Treatment of certain wholly owned subsidiaries
(A) In generalExcept as provided in regulations prescribed by the Secretary, for purposes of this title—
(i) a corporation which is a qualified subchapter S subsidiary shall not be treated as a separate corporation, and
(ii) all assets, liabilities, and items of income, deduction, and credit of a qualified subchapter S subsidiary shall be treated as assets, liabilities, and such items (as the case may be) of the S corporation.

B) has c corp earnings and profits and has too much passive income for 3 consecutive years

NON AUTOMATIC

 The election is revoked. An election can be revoked only with the consent of shareholders who, at the time the revocation is made, hold more than 50% of the number of issued and outstanding shares of stock (including nonvoting stock). The revocation can specify an effective revocation date that is on or after the day the revocation is filed. If no date is specified, the revocation is effective at the start of the tax year if the revocation is made on or before the 15th day of the 3rd month of that tax year.     If no date is specified and the revocation is made after the 15th day of the 3rd month of the tax year, the revocation is effective at the start of the next tax year.
To revoke the election, the corporation must file a statement with the appropriate service center listed under Where To File in the Instructions for Form 2553. In the statement, the corporation must notify the
IRS that it is revoking its election to be an S corporation. The statement must be signed by each shareholder who consents to the revocation and contain the information required by Regulations
section 1.1362-6(a)(3).

you should also consult a tax pro.  there may be adverse tax consequences when you convert.  

also  why do you want to convert?  Under certain circumstances a C-corp is the worst possible tax structure.      again seek the advice of a pro.        

I will also advocate getting some professional input on any change.

Not good to just knee jerk because the tax rate "appears" lower.  Keep in mind that tax laws change annually.

Additionally, long term, the S corp builds up basis in your investment which could lead to less tax upon disposition.  Also the ability to take out tax-free distributions to the extent you have basis AND are taking a reasonable salary.

You don't "change" anything. The S-Corp is closed permanently and forever with a "final" 1120-S and final K-1's are issued to all owners. Then a "new" business that is a C-Corp is opened one day after the S-Corp is closed.

Now you "REALLY" have to know what you're doing to get dates right too. Otherwise you'll have major, major, major issues with transferring assets and inventory.