Update fail... - Lite Source Repository and Ubuntu Repository change

[Şerban S.]

[...]  Err :1 https://linuxlite.3load.com fluorite InRelease

Here, you need to change the repository. For whatever reason, the server is either on maintainance, or has removed the Linux Lite mirror.
Search for the best server and change it.
If you need further instructions, ask in a new post, after updating.

[...]  Ign :9 http://linux.dropbox.com/ubuntu disco InRelease

You need to disable the above PPA. They do not have the appropriate version for the OS.

[...]  E: Impossible de récupérer http://linuxlite.3load.com/dists/fluorite/InRelease  403  Forbidden [IP : 188.114.97.2 443] [...] 

This message is likely triggered by the above errors.
Try again to update after disabling the DropBox PPA. The update should work.
If you really need DropBox, install it through Lite Software, after the update.
But first, change your source repository. Might just be too slow, it's difficult to know what exactly triggers the 403 error.

Best regards, Șerban.