You can search the forum
The long story:
https://openwrt.org/docs/guide-user/additional-software/extroot_configuration
The short one:
First reset the router so that you have more space.
- Use a USB disk, format as ext4
- plugin the USB disk to the router
- Install luci, go to luci and change the mount point /overlay to your USB
- Copy your current system to the USB
- Reboot