If you only want to display the hexadecimal content of a string, write it in an indicator and select the hexadecimal display with the right button menu.
If you need to convert the string, tt can be done using 3 LabVIEW functions, no loop:
ASCII String->String to Byte Array->Number to Hexadecimal String(length 2)->Concatenate Strings->Hexadecimal String.