I was unable to update to Ubuntu 20 from Ubuntu 18.04

You have to remove third party repository. You can do it using one of two method:

Using the GUI (Software & Updates):

1. Open Software & Updates app.
2. Switch to Other Software tab.
3. Uncheck all checkboxes (it will prompt for password first time).
4. Retry upgrade to 20.04.

Using terminal:

1. Issue this command on terminal:

   cd /etc/apt/sources.list.d
   

2. Using an editor like nano or vi comment all uncommented line (put a # character at the beginning of line) in all files. (You need to use sudo) (You have to open all files one by one) (Uncommented line indicates those lines which don't start with a # character.)

3. Retry upgrade to 20.04.

Just

• Go to Software & Updates
• Go to Ubuntu software
• In the Download from menu select main server

Then try to update Ubuntu. It worked very well for me.