/usr/syno/synoman/webman/modules/FileBrowserand the file is FileRequest.js. Inside you will find "Copyright @ 2019 Synology Inc." divided into variables (just search for it). Edit it, save and reboot (it won't work if you just save the file and reload, you can try).
Hmm guess there has to be something that’s blocking to change that or there is another file that controls this. I’ll have to dig up a bit more. Was working late yesterday so did this with half brain .That's what I did before and it worked. However, it does not work with the current version of DSM. Always displays "Copyright @ 2019 Synology Inc." no matter what I replaced it with. Reload, reboot, etc. There must be a master file to prevent editing?