What should I fix ?

You have to always provide the parameters defined in the query string. Because you are updating a recordset and don't want to modify all fields, you have to use different SQL command strings.

You can use a sub string for that:

SqlCommand cmd = conn.CreateCommand();
cmd.Parameters.Add("@CustID", SqlDbType.Int).Value = Convert.ToInt32(cust.ToString());
String queryParams;
if (ULN.Text != "")
{
    queryParams = "LastName=@LastName"; 
    cmd.Parameters.Add("@LastName", SqlDbType.VarChar).Value = ULN.Text; 
}
if (UAddress.Text != "") 
{ 
    if (queryParams != "")
        queryParams += ","; 
    queryParams += "Address=@Address"; 
    cmd.Parameters.Add("@Address", SqlDbType.VarChar).Value = UAddress.Text; 
}
if (UPhone.Text != "") 
{ 
    if (queryParams != "")
        queryParams += ","; 
    queryParams += "phone=@phone"; 
    cmd.Parameters.Add("@phone", SqlDbType.Int).Value = Convert.ToInt32(UPhone.ToString());
}
cmd.CommandText = "UPDATE Customers SET " + queryParams + " WHERE CustID=@CustID";

ULN.Text is null, that's why you're getting that error. You need to add defaults to your parameters, or build your SQL query at runtime. I suggest the null-resolution operator rather than if statements.

I also suggest the "using" keyword to ensure proper disposal and avoid memory leaks.

var ConnectionString = "Data Source=DESKTOP-411ATJG\\SQLEXPRESS; Initial Catalog=CarRental; Integrated Security=True";
var SQL = "Update Customers SET LastName=@LastName,phone=@phone,Address=@Address where CustID=@CustID";

using(SqlConnection conn = new SqlConnection(ConnectionString)){
   using(SqlCommand cmd = conn.CreateCommand())
   {
      cmd.CommandText = SQL;
      cmd.Parameters.Add("@CustID", SqlDbType.Int).Value = 
Convert.ToInt32(cust.ToString());
      cmd.Parameters.Add("@LastName", SqlDbType.VarChar).Value = ULN.Text ?? string.Empty; 
      cmd.Parameters.Add("@Address", SqlDbType.VarChar).Value = UAddress.Text ?? string.Empty; 
      cmd.Parameters.Add("@phone", SqlDbType.Int).Value = UPhone.Text != null ? Convert.ToInt32(UPhone.Text.ToString()) : 0; 
      conn.Open();
      cmd.ExecuteNonQuery();
   }
}