Yes, you can come back later and update your 2018 tax vouchers. The 1st quarter voucher will be due in April. After you file your taxes online, you will need to 'open' up the return again to revisit the estimated tax section.
When you revisit: Log into your account, scroll down a bit and select, 'add a
state'. You aren't really adding a state, just opening up the return. Then
navigate to the W-4 and estimated tax section under Other Tax Situations. Then print from the Print Center under Tax Tools.
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