The correct expressions are 'in detail' and 'in more detail'. I have never heard a native speaker say 'in details'.

I always say: "let's look in details", but you can say too: "let's look in detail" or "let's look in more details". You can use the online dictionary: context.reverso.net I often use it when I watch a film or anything. It helps me a lot. I hope my answer is useful for you! :)