No the W-2 cannot be changed because it reports what was actually contributed.
If returned in 2019 then yiu should have received a 2019 1099-R with a code P in box 7 that would be reported as wages in your 2018 tax return because you must claim the amount that was not reported on the W-2 as wages as if they were on the W-2.
Any earnings attributed to the excess would be reported on a 2nd 1099-R with a code 8 in box 7 that is taxable in the year returned - 2019, so that goes in yiur 2019 tax return.
If you did not receive the 1099-R's then contact the 401(k) plan trustee as there were required by law to issue them no later than Jan 31, 2020.
For information see IRS Pub 525 page 10
https://www.irs.gov/pub/irs-pdf/p525.pdf
**Disclaimer: This post is for discussion purposes only and is NOT tax advice. The author takes no responsibility for the accuracy of any information in this post.**
