This question already has an answer here:
How to find sub-string and replace it?
Barring additional information it's impossible to know if the following is adequate, but it works for your sample data:
SELECT REPLACE([Column-2],'- x','- ZZ x') FROM YourTable
SELECT STUFF([Column-2],CHARINDEX('-',[Column-2]) + 1,1,' ZZ ') FROM [table_name]