Where to start?! More details would help. Are you using WiFi? Is the connection/speed good? Are you using "DJI Assistant for Mavic 2" on your computer, or are you going wireless through a Smart Controller? Do you have the drone turned on and connected to the controller? Are the batteries well over 50% on both? Did it hang up at 90 something percent and did you wait a good while? And I'm sure there are other questions I'm forgetting . . . smile!
Welcome to the forum. When you have technical issues such as firmware update failures I have found it helpful to call DJI support. They are pretty responsive most times... if you get a knowledgeable support technician.
Welcome to the forum, try using DJI Assistant 2 on the PC to do the firmware upgrade.
Another trick is to connect the PC via a Ethernet cable directly to your router/modem, sometimes flaky wi-fi can cause the firmware upgrade to stall.
Welcome to the forum.
I hope you will find our site helpful and look forward to any input , photo's/video's you might post .
Don't be shy and ask anything if you can't find it by searching.