I've decided that I don't want to defer part of my social sec payment. How do I change line 18 of sch se to 0?

You can reverse/cancel your election to defer your self-employment taxes. 

Please follow these steps in TurboTax to change your election:

1. Open your return.
2. Click Federal on the left hand side. 
3. Click Deductions and Credits on the top of the page.
4. On the page Tax Breaks, scroll down until you see Tax relief related to Covid-19.
5. Click Revisit next to Self-employment tax deferral. 
6. Answer no to the question Do you want more time to pay self-employment taxes? 

This question allows half of the SE attributed to income during COVID to be deferred.

Self-Employment tax (Scheduled SE) is automatically generated if a person has $400 or more of net profit from self-employment.  You pay 15.3% SE tax on 92.35% of your Net Profit greater than $400.  The 15.3% self-employed SE Tax is to pay both the employer part and employee part of Social Security and Medicare.  So you get social security credit for it when you retire.