First you need to buy a service manual. Right now, there are two on Ebay for 1967 6HP Evinrudes, which are the same motors as yours with different paint, decals and covers. Or, you can get a new one from one of several companies that sell them including maxrules .com, marineengine .com, kencook .com, and others.
Then you can get help here if you run into difficulty.
You might also find something that will help among the thousands of threads on this forum if you try the search function.
Changing the lu oil is a bit easier. To start you will need some oil and a pump. Recommended is 80-90 outboard gear oil, it has additives to protect the lower if you have water intrusion. Always replace the washer on the vent and drain screw, as this is the #1 cause of water intrusion. On the older electric shift motors, you need Premium Blend Type C for electric shift. Remove both the drain and vent screw, letting the oil drain. Install the new washers. Hook the pump up to the drain screw, and fill the gearcase until the oil starts to exit via the vent hole. Replace the vent screw, this will create a vaccuum, allowing you to remove the pump and install the drain screw. If the oil looks milky or white, let us know, that means there is water in the oil and further measures will need to be taken. Thats it, all done.
Here is the pump that was referred to, not necessary, but you will be thankful once you have it, or try the tube method first.