TL;DR
In case you have a problem with Booking.com, a call to their customer service might just resolve the issue.
The long story
I have an upcoming trip from Guadalajara to Newark. The best itinerary (among a lot of bad options) involved a 6:30 am departure heading north, and an overnight layover at DFW on the way home. So I booked two hotels, one at GDL for the night before my departure and one at DFW on the way home. I got the best rates in both cases at Booking.com. The GDL hotel (the brand new Hilton Garden Inn right at the airport) was a non-refundable rate.
To my horror, I discovered a bit later (after importing my confirmation emails to Wanderlog) that both hotels were booked on the same dates. I believe what happened is I booked the DFW hotel first, then when I booked the GDL hotel, Booking.com remembered the dates from my previous search, and somehow those are the dates I booked. Or it's possible I just wasn't observant .
Using the Booking.com website, I requested a date change. The messaging from Booking.com said have sent the request directly to the hotel, and the hotel will have to approve it.
A couple of days went by and there was no response from the hotel. I messaged the hotel through the Booking.com messaging interface, and they responded that they would be happy to make the change as soon as they receive it from Booking.com. I sent a second request on the Booking.com website, and still no response from the hotel.
So I called the hotel directly, and they found my reservation, and they could see the message I sent, but they had never received a request for the date change from Booking.com.
So I called Booking.com after hunting all over their website to find the customer service phone number. My call was answered by an AI bot who seemed quite friendly. She took my information and transferred me to a live person, and I immediately got cut off. I called back and chatted with the AI bot again, and this time when she transferred me, it went quickly to a real person. She took all my information and said she would call the hotel to get their approval on the date change. She told me the rate for the new date would be higher. I said I didn't think that was right based on the information I got on their website. So she went ahead and called the hotel, and when she came back, she said the hotel approved the date change at the same price. (Normally, she said, any discounts I received as a VIP Booking.com member would not apply to the change.)
So don't despair if you have a problem with Booking.com. But be extra careful about the dates!