Limited space routers.
AGH binary takes up 35mb and during its update it will download the new binary compressed, backup your old binary and then extract and replace the old binary with the new one. This means you require 35mb x2 and the compressed file space and extraction space.
To manually do this, do an upgrade from the control panel and look for what version it wants to upgrade to in your syslog.
AdGuardHome[5830]: 2021/11/10 16:03:30.149784 [info] Updating from v0.107.0-a.199+2fc10848 to v0.107.0-a.203+6fd9e72f. URL:https://static.adguard.com/adguardhome/edge/AdGuardHome_linux_mips_softfloat.tar.gz
SSH into your router and stop AGH.
/etc/init.d/AdGuardHome stop
download the new version to your routers /tmp area or to your laptop/pc. Then either unpack AGH binary from the compressed download over the existing binary or unpack the binary on your laptop/pc and copy it over the existing binary using something like WinSCP or equivalent.
Now restart AGH.
/etc/init.d/AdGuardHome start
(Edit) commands to do this below.
#Stop AGH
/etc/init.d/AdGuardHome stop
#Grab updated AGH from server and save to /tmp
wget https://static.adguard.com/adguardhome/edge/AdGuardHome_linux_mips_softfloat.tar.gz -P /tmp
#unzip updated file over top of AGH in /opt
tar x -vzf /tmp/AdGuardHome_linux_mips_softfloat.tar.gz -C /opt
#cleanup /tmp
rm /tmp/AdGuardHome_linux_mips_softfloat.tar.gz
#Restart AGH
/etc/init.d/AdGuardHome start