Convert SQL query to Linq query in ASP.Net

Refer the below link how to work with entity framework

Simple Entity Framework Tutorial in ASP.Net Web Forms with example

Also check the below sample example and implement it as per your code logic.

int Id = 10;
var empId = db.tbl_EmpInfo.Where(x => x.EmployeeID < Id).OrderByDescending(x => x.EmployeeID).Select(x => x.EmployeeID).FirstOrDefault();