First configure the server on LibreELEC:
From a terminal edit aria2.conf:
nano /storage/.kodi/userdata/addon_data/service.aria2/aria2.conf
Adding the following line should be enough for a start:
dir=/storage/downloads
Then restart the service:
systemctl restart service.aria2
Then, on a system which has a browser with which you want to access aria2:
Download and unzip https://github.com/ziahamza/webui-aria2/archive/master.zip
Open webui-aria2-masterdocs/index.js with the browser
Configure 'Connection Settings' (setting the host to the IP address of your LibreELEC device should be enough).