Downgrade R version 4.0 to R version 3.6

I have encountered problem working with R version 4.0 and wanted to downgrade to R version 3.6. Here is a way how to do this:

1. Remove the currently installed R version using the following commands:

   sudo apt-get remove r-base-core
   sudo apt-get autoremove 

2. Find the version that you need by running the following command:

   apt-cache showpkg r-base

The output should look something like this: 

Provides: 
4.0.2-1.1804.0 - 
4.0.1-1.1804.0 - 
4.0.0-1.1804.0 - 
3.6.3-1bionic - 
3.6.2-1bionic - 
3.6.1-3bionic - 
3.6.1-1bionic - 
3.6.0-2bionic - 
3.6.0-1bionic - 
3.5.3-1bionic - 
3.5.2-1bionic - 
3.5.1-2bionic - 
3.5.1-1bionic - 
3.5.0-1bionic - 
3.4.4-1ubuntu1 - 

So, I want to install 3.6.3-1bionic version. This can be achieved using the following command:

sudo apt-get install r-base-core=3.6.3-1bionic