App not working. Stuck on downloading update... and nothing is happening.

Try disabling firewall and antivirus software and see if that helps.

I did. Everything was working in last week, and now update is not uploading.

Just got error message "Network error while fetching difference list: NetworkSendError"

Yes this is common error message in case of firewall/antivirus is blocking the connection.
You can also try different ISP if possible.
And try disabling firewall on router/modem.