The installer itself doesn't leave a version anywhere on the system. You can, however, check the version of U-Boot which created the environment on first-boot, which is hence the version which came with the installer:
fw_printenv ver
To update the recovery image written by the installer, first make sure you have a working production system (to have a fall-back in case something goes wrong). Note that you will loose all configuration as the new recovery image may need more space as the old one and hence rootfs_data needs to be removed during the procedure (it is re-created by U-Boot on reboot).
Boot into recovery and do this:
cd /tmp
wget https://downloads.openwrt.org/snapshots/targets/mediatek/mt7622/openwrt-mediatek-mt7622-linksys_e8450-ubi-initramfs-recovery.itb
ubirmvol /dev/ubi0 -N recovery
ubirmvol /dev/ubi0 -N rootfs_data
ubimkvol /dev/ubi0 -N recovery -s $(cat openwrt-mediatek-mt7622-linksys_e8450-ubi-initramfs-recovery.itb | wc -c)
ubiupdatevol /dev/ubi0_2 openwrt-mediatek-mt7622-linksys_e8450-ubi-initramfs-recovery.itb
reboot
If you really want to update U-Boot and/or TF-A, you can do so with the serial console attached (there are options for updating in the boot menu) or by writing it using mtd and kmod-mtd-rw. There is no fallback, so if something goes wrong, you have to connect JTAG and have fun with OpenOCD.
insmod mtd-rw i_want_a_brick=1
# to update bl2/Preloader (should never be needed unless you wanna switch to home-brew secure-boot system ;)
cd /tmp
wget https://downloads.openwrt.org/snapshots/targets/mediatek/mt7622/openwrt-mediatek-mt7622-linksys_e8450-ubi-preloader.bin
mtd write openwrt-mediatek-mt7622-linksys_e8450-ubi-preloader.bin /dev/mtd0
# to update U-Boot and ARM Trusted Firmware BL31
cd /tmp
wget https://downloads.openwrt.org/snapshots/targets/mediatek/mt7622/openwrt-mediatek-mt7622-linksys_e8450-ubi-bl31-uboot.fip
mtd write openwrt-mediatek-mt7622-linksys_e8450-ubi-bl31-uboot.fip /dev/mtd1
# clear ubootenv and ubootenv2 UBI volumes, so environment is re-created by updated U-Boot
ubiupdatevol -t /dev/ubi0_0
ubiupdatevol -t /dev/ubi0_1
reboot
Do not re-run the installer, as you will loose your backups of the vendor bootchain.