I highly suggest making a backup of your router configuration first. Here's a good reason to use some basic shell script usage, SSH & SFTP (Secure FTP (File Transfer Protocol) — FTP using a SSH (Secure SHell) tunnel):
It won't back up your OpenSpeedTest installation but that's easy enough to reinstall if needed. It's always good to have a 'fall back' plan before one gets all 'experimental'... especially when exploring OpenWrt's LuCI GUI. 
You may want to take a look @ FileZilla Client (SFTP, FTP, FTP/S)(multi-platform) & MobaXterm (SSH, SFTP)(Windows). They're free. The SFTP server/daemon (openssh-sftp-server will be listening for LAN-only connections on port 22 once installed. It's the same username/password as logging into the GL GUI but with the user name of 'root' — the default administrator account name on all/most Linux operating systems. It's also the same login for LuCI.