There are bits & pieces all over the web to do this, but all the dots aren't connected. This is tested & works:
-Use either Edge or Chrome for this with a tested good USB-A to USB-C (or on new laptops USB-C to USB-C)
-Insure the phone is 50% or better charged & laptop is on AC: flashing but losing power in the process will brick the phone.
-Download the USB driver from: https://flash.android.com/
-Open device manager, & plug in the phone cable connected to your laptop.
-Look for what changes in device manager upon refresh (mine said "ACER ADB interface"
Right-click on it & select "update driver". After you've extracted the driver, go to that directory from "update driver' & select.
Once it says successful, reboot phone & laptop, & opening device manager on your laptop. If all went correctly, the "ACER ADB interface" should now say "Android device".
===> Backup all data you want NOW from your phone as it will be wiped with the 2nd command below.<===
-Open a command line as an administrator, and type "fastboot devices"
If all is running right, it should list your phone's serial number + fastboot
-Type: fastboot flashing unlock
-Wait until the phone says "unlocked"
-At this point, go to link: https://grapheneos.org/install/web#replacing-grapheneos-with-the-stock-os
and select the last item listed as "remove non-stock key"
-On your laptop it should say "key erased" if successful
-Next, got to weblink: https://flash.android.com/
-Select device (Pixel 7) & connect
-Select "Back to Public"
-And install build (in this case 15.0.0)
-And confirm
-Allow access, & accept license
-Download begins
-Reboots a few times (you'll see various sizes of data loading (don't touch)
Lock bootloader (Up or down power button until "lock
bootloader" is seen & select.)
Phone reboots into Android 15.0.0