tickets/tomorrow night/ Mon Sept 11

You can call the box office and tell them you won't be able to make it. If you have room, they may offer to put you in on another date. It's worth a try. It's called past dating.

I found this in a playbill.com article from 2017, maybe it will help. I believe Six is a nederlander theatre, it was formerly the Brooks Atkinson only renamed recently:

A spokesperson for the Nederlander Organization — which controls the Nederlander, Brooks Atkinson, Lunt-Fontanne, Gershwin, Marquis, Minskoff, Neil Simon, Palace and Richard Rogers Theatres — says that, as with Jujamcyn, if you miss a performance at a Nederlander theatre, you can call the theatre on the day of the performance that you would like to see instead. The substitute performance must be Monday through Thursday. If the performance isn't sold out, the theatre will accommodate you. To inquire, you should call the general number for the individual theatre, which is listed in the White Pages. If you're really stuck you can call the organization's general number at (212) 840-5577.

You can fill out a form and submit online after you miss the show and they will help you past-date the performance.

https://help.broadwaydirect.com/hc/en-us/articles/1500010121141-How-do-I-exchange-or-refund-my-tickets-