Update data from one table to another table?

Question

I have two table both the tables are having same columns.

 

In first table I have Mobiile_No column where there is no null value inside column.

 

But in second table Mobile_No column has few null values. like some users Mobile no is not their,  but its there in first table.

 

what should be the query to update Mobile_No column from table one to table two.

assuming that join should be EmployeeId