Yes, users can modify an existing reservation on OpenTable. To do this, one should navigate to the OpenTable website or mobile application and sign into their account. Once logged in, the user can locate the “Upcoming Reservations” section, where their current reservations will be displayed. By selecting the reservation they wish to modify, they will typically find options to change the date, time, number of guests, or restaurant. It is important to note that any modifications are subject to the availability of the restaurant. Additionally, if a user encounters any issues or if they need to make significant changes that cannot be adjusted online, it might be best to check the specific restaurant's policy regarding modifications. For detailed instructions, users may also refer to the relevant sections on the OpenTable website.