Is it possible to download the update manually and update it by means of using FTP server?

If you want to update the base module, but you can't access the app update through the front end, you can try this code in the backend to update the base module automatically by reloading the front end.

in your odoo conf add the code like this:

-c odoo.conf -u base

then run the server > then refresh the front end so that the base module will be updated.