What's a FIN? Do you mean EIN (Employer Identification Number) or ITIN? (Individual Taxpayer Identification Number).

It's the Federal Identification No on 1099R and, I guess, same as EIN

I had a typo-error and transposed 2 numbers  of my FIN and the returned was rejected. I need to make the correction and re-e-file it

Do it right the first time. Otherwise, you risk delaying your refund. The feds will NOT look anything up "for you" at this time of year. They just flag it, put it in  the "take care of this later" pile and there it sits until tax season is over.
To correct the 1099-R under the Personal Income tab scroll down to the Retirement Income & Social Security section and elect to update IRA, 401(k) Retirement income (1099-R). Indicate that you DO have a 1099-R and you'll get to the screen that will list your 1099-R. Elect to edit that 1099-R and work it through, making corrections where necessary. Changes don't "stick" until you click a DONE button. So keep working through until you get to a screen with a DONE button on it, and click it.

EIN/FIN is the same number. As long as you got your name, address, and social security number entered correctly, the Feds should still approve you, assuming you don't have any other red flags trigger your return. They can easily lookup your employers EIN/FIN. Companies with over 200 employees have a public EIN/FIN number. You may be able to Google it if so.

If not, wait for that W-2 or call your employer or there Payroll Company even.

I need to change FIN 2020 Tax Return I already processed.

You may need to amend your return if it becomes rejected from the IRS after it was transmitted.  To do this, please see the link here for further instructions.