l am scheduled for a car test. During pre-check the mechanic told me that the lambda should be replaced since the emission is above the limit. l just changed my oil last month, is there really a need to replace the lambda.? How much is the average cost of the lambda in euro?

Depends on the exact model, but they're £35 (about €50) from here;
http://onlineautomotive.co.uk/

If it needs replacing, then there is nothing you can do - they are prone to failing after a period of time.