Well... different ways of case numeration are possible. So "sixth" may be Prepositional case or Instrumental case or.... As for "something being somewhere", it most probably denotes Locative case, which usually (but not always) coincide in form with Prepositional case and normally is not listed among standard Russian cases. So, I assume, you need Prepositional case.

If you speak about changing form of nouns, adjectives etc. according to the case, it is called "to decline". Term "to conjugate" relates to verbs. This also may be important when you use search.

BTW, in grammar textbooks rules of declining nouns are normally grouped not by cases but by declensions (groups of nouns, which decline similarly).

In addition for each word class that can be declined (nouns, adjectives etc.) there are different rules. So when you formulate request for search or question in forum, it should be specified, which word class exactly you want to decline.

Now if you need rules for the Prepositional case of nouns, you can google and find, for example, this.