I can not find the
autocompletion.plist in the Texshop 3.18. Is there another way to redefine the autocompletion for this version?
First, do you mean AutoCompletion or Command Completion. I assume you mean AutoCompletion which is now called Key Bindings (which describes the feature better). In newer versions of TeXShop (it's now at 3.51) you can edit the Key Bindings file using the Source->Key Bindings->Edit Key Bindings File… Menu item.