I'm in the same boat as you, ready to upgrade my x86/64 router from 21.02.0 to 21.02.2. The relevant portion of the page @RaylynnKnight linked is the following:
If you had used a **ext4-combined.img.gz** type of image to install, there are 4 options for upgrading:
1. Write a new **ext4-combined.img.gz** image: this is the simplest option and is identical to first installation: all data, configs, packages and extra partitions will be wiped and you'll have a brand new OpenWrt system with default packages and configs. Then you can reinstall all packages and copy config files back and create extra partitions.
2. Use **sysupgrade**: this is default upgrading procedure, but the least recommended option for x86 machines. Proceed to [Sysupgrade](https://openwrt.org/docs/guide-user/installation/installation_methods/sysupgrade) for details.
3. Extracting boot partition image from **ext4-combined.img.gz** and writing it and **ext4-rootfs.img.gz**, leaving MBR partition table intact.
4. Extracting boot partition image from **ext4-combined.img.gz** and writing it, then uncompressing **rootfs.tar.gz** to existing rootfs partition.
The frustrating part is #2. It recommends against using sysupgrade but doesn't explain why. For myself, I'm going with #1, which is basically a re-install but I intend to use the Image Builder this time to hopefully skip the extra package loads and reconfiguration.
Take a look at: Using the Image Builder
The linked page has very good instructions for starting from the image you want to upgrade to but adding all of the packages you are currently using as well as preserving your current config. I think it is kosher to include your current config files if your doing a minor "point" upgrade such as 21.02.0 to 21.02.2.
I was able to successfully build my image with extra packages and configs. I created a bootable USB drive with Ubuntu BUT also with persistent storage so I could copy my newly built image to the USB drive. This will let me boot into the live Ubuntu image and then dd the custom OpenWRT image to my router.
Now I just need to find a time that both my wife and son don't need the network for about an hour. The good thing about installing on a x86 computer is that you really can't screw it up (brick it). Worst case is just a full install and manual update and config like you did the first time.
Good luck.