I'm not sure what you mean by "we" overcontributed. 401(k) contributions are individual contributions.
You must do what the HR rep said. Obtain the necessary form from the HR rep or the plan administrator, complete the form and submit it as instructed. Doing this soon enough so that the correction is made before the end of the year will greatly simplify the process when it comes time to prepare your 2023 tax return. Failure to make the correction will result in double taxation of the $1,400.
Overcontribution should only have been possible if you had more than one employer in 2023 and contributed to multiple 401(k) plans, otherwise the plan should have automatically stopped taking elective deferrals from your paycheck. Make sure that you are not confusing 401(k) contributions (with a regular elective deferral limit of $22,500 for 2023) with IRA contributions. A 401(k) is not an IRA.
