Conditional logic checks on existing column

I have created a new column in table B called Dashboard Country which was created from merging table A and table B on Support ID.

I now have dashboard country on table B populated with the relevant countries. However I now need to apply more conditional logic to this column to get rid of the null values. 

Is there a way to apply conditional logic to an existing column? If not how do I create a new column that keeps all the non null values in it?

The addtional logic checks i need to do on the null values are "if column "a" contains "string" then change from null to "new country".

The end result would be the countries from the original merge would be there and the nulls would change based on the additional if statement I want to apply to the nulls. 

Table A:

Table B

Thanks