I click the full upgrade instructions under member.php and it brings me to http://www.amember.com/forum/showthread.php?t=3133. Then this post tells me to go to member.php. It's bouncing me back and forth. Where is the upgrade doc for 3.x.x to 3.x.x (3.1.6 to 3.1.8).
Go to member.php page and just above where you select your version to upgrage to 3.1.8 is the start of the upgrade instructions. Just click where it says 'Read More'.