If then statement change existing column

How can I do this. I tried a case statement but have issues getting it to work

if Tax_or_Flat = 'tax' then flata = 0

Tax_or_Flat is a column title and flata is a column title

maybe ,CASE WHEN Tax_or_Flat = 'tax' THEN Flat = 0 ELSE Tax_or_Flat END

but thats not working