Implement Logic For Update Or Insert

I have a stored procedure that I need to either perform an update if a record exists or an insert if the record doesn't exist.

Here is my procedure:

CREATE PROCEDURE dbo.Insert_Temp_ContactInfo

@sessionid varchar(50),
@FirstName varchar(50),
@LastName varchar(50),
@SchoolName varchar(50),
@address varchar(50),
@City varchar(50),
@State int,
@Zip varchar(5),
@Phone varchar(10),
@Email varchar(50),
@CurrentCustomer varchar(3),
@ImplementationType int,
@ProductType int = null,
@Comment varchar(500)

AS

--check if a current record exists

SET NOCOUNT ON

SELECT FirstName, LastName, SchoolName, Address, City, State, Zip, Phone, Email, CurrentCustomer, ImplementationType, ProductType, Comment
FROM dbo.Temp_ContactInfo
WHERE sessionid = @sessionid


SET NOCOUNT OFF


--if exists update the record

UPDATE dbo.Temp_ContactInfo
SET
FirstName = @FirstName,
LastName = @LastName,
SchoolName = @SchoolName,
Address = @address,
City = @City,
State = @State,
Zip = @Zip,
Phone = @Phone,
Email = @Email,
CurrentCustomer = @CurrentCustomer,
ImplementationType = @ImplementationType,
ProductType = @ProductType,
Comment = @Comment
WHERE sessionid = @sessionid

--if the record does not exist, then insert a new record

INSERT INTO dbo.Temp_ContactInfo (sessionid, FirstName, LastName, SchoolName, address, City, State, Zip, Phone, Email, CurrentCustomer, ImplementationType, ProductType, Comment)
VALUES (@sessionid, @FirstName, @LastName, @SchoolName, @address, @City, @State, @Zip, @Phone, @Email, @CurrentCustomer, @ImplementationType, @ProductType, @Comment)

GO

I think I can use an if statement. Can anyone help me out with this?

Thanks.