How to extend and convert a conference paper into a journal submission? I have been asked to develop a extended version of an accepted conference paper. This extended version will be submitted to a journal for review. 

Please tell me how to develop the same. Is there any need to provide complete/partial results in this extended version?

Is it not only sufficient to further improve the idea instead of presenting the results?
This varies from journal to journal. For example, in some subfields of CS (where this comes up all the time), the informal rule is that the journal version should have at least 30% new content (including new experiments, new algorithms and so on), and this new content should be identified clearly in a cover letter. In other subfields (like in theory) the expectation is that all proofs will be presented in full detail (no sketches), but new results are not necessary. 

So it depends greatly on the subfield and the journal. As always, when in doubt, ask the editor of the journal, or colleagues who've submitted there before.