You are missing @Email parameter in the list. Declare a variable to hold the value of Id.
ALTER PROCEDURE [dbo].[insert_FBSignUp]
@name nvarchar(200),
@uname nvarchar(200),
@pwd nvarchar(50),
@gender nvarchar(10),
@Email nvarchar(200)
AS
BEGIN
DECLARE @ID int
Select @ID = id from dbo.tblUsers where Username=@Email
UPDATE dbo.tblUsers
SET (uname = @uname,
pwd = @pwd,
gender = @gender)
WHERE id = @ID