Yes, you should select to pay by check, then use the information below to revise your installment agreement.  

You can request an amendment to the installment agreement by using this link.  Apply/Revise and installment agreement online.

Request by calling the IRS at 1-800-829-7650. You can completing Form 9465 with information about both the original agreement balance and the expected new balance.

• Note: Setup fees may be higher if you apply for a payment plan by phone, mail, or in-person. Get more information on other payment plan options and fees.

Key Points:

1. The IRS does want you to have a plan that would pay as much as possible, however make your requested payment amount you can afford, because you don't want to miss a payment.  You can always pay more when you have it.
2. Penalties and interest continue to accrue until your balance is paid in full. 
3. Your future refunds will be applied to your tax debt until it is paid in full.
4. Make all scheduled payments even if we apply your refund to your account balance.
5. Check out the definition of 'Lower Income Taxpayers' (link above or Form 9465 Instructions)

Hi,

Same situation here

I already file and choose pay by check. Now if I want to consolidate the tax balance online ( combine the 2019 and 2020). Can I do it right away? Or how long should I wait? Right now I don't think I can do anything through online portal. I only can revise the method of payment, amount I want to pay per month and the date.

read the instructions on the IRS website carefully.

you have to request to have your new tax due merged into an existing payment plan. this may or may not be approved.