how to make an order button to a buy now button.

Are you using the Booking API or the Stores API?

If you are using the Booking API, you can do this:

import wixBookings from "wix-bookings"

$w.onReady(async () => {
    let availability = await wixBookings.getServiceAvailability("87edd4e0-42b1-4802-8766-584f3eeb6436") //Service ID
    let slots = availability.slots
    let chosenSlot = slots[1] //Chosen slot available to booking

    let formFieldValues = [
        {
            _id: "20657271-c55f-43d6-adfd-39b7acc38e11", // name field ID
            value: "John Doe",
        },
        {
            _id: "87edd4e0-42b1-4802-8766-584f3eeb6436", // email field ID
            value: "john@doe.com",
        },
    ]

    let bookingInfo = {
        slot: chosenSlot,
        formFields: formFieldValues,
    }

    let options = {
        paymentType: "wixPay_Offline", //Here you can define the payment method
        couponCode: "thecouponcode", //Here you put the coupon code
    }

    let booking = await wixBookings.checkoutBooking(bookingInfo, options)
    let bookingId = booking.bookingId
    let bookingStatus = booking.status
    console.log(bookingId, bookingStatus)
})