Can I use my old W2 (2019) to help file this year's taxes. Will the information be different if I only worked 3 months

[VolvoGirl]

No.  You need a 2020 W2. Everything will be different.

[DoninGA]

A 2019 W-2 cannot be entered on a 2020 tax return.

To enter the 2019 W-2 you will have to amend the 2019 tax return that you filed.

See this TurboTax support FAQ for amending a 2019 tax return originally completed and filed using TurboTax - https://ttlc.intuit.com/community/tax-topics/help/how-do-i-amend-a-2019-return-in-turbotax/01/123845...