How to add leaves Every month against Employee in using SQL Job

I see a problem with your database design. Instead of storing 'Remaining leave' in a database column, you should ideally calculate it every time the value is required based on the current date and month.

If you still wish to proceed with your design, I would suggest you create a stored procedure that updates the leave balance and call this stored procedure from an SQL job that runs on specific intervals. Your logic can still be wrong if the stored procedure is run more than once during the same interval, unless of course you take precautions to avoid multiple updates to leave balances.