Try this:
DROP TABLE IF EXISTS #temp;
CREATE table #temp ( employee varchar(100), designation varchar(100))
insert into #temp values ('AAA','IT Sales Person'),('AAA','Dept head'),('AAA','supervisor')
insert into #temp values ('BBBB','Sales Manager - Area'),('BBBB','Dept head')
insert into #temp values ('CCCC','Sales Manager - Region') , ('CCCC','Dept head')
SELECT employee, designation, CASE designation WHEN 'IT Sales Person' THEN 'Salesperson'
WHEN 'Sales Manager - Area' THEN 'Sales Manager' WHEN 'Sales Manager - Region' THEN 'Salesperson' ELSE designation END AS [New Designation]
FROM #temp