welcome to the garage Tony - we'll help ya thru it!
detailed instructions:
- completely remove heater assembly from truck, making careful note of the pieces removed and their location
- dissassemble the box to find the offending part inside, again noteing pieces removed and their location
- reverse the process using the new part and the notes made
- attach the hoses to the heater
come back and ask further questions, like 'why's it leaking' or 'do these heaters actually heat' and we'll sort it out w/ ya
otherwise, it's prolly covered in the shop manual, and buying one is usually cheaper than paying a mechanic to read it
Bill