Hi Shruti,
You have not mentioned your board here . If your board is CBSE then improvement exam of CBSE is conducted along with final exams i.e. in march not in July.

You don't have to give practical examination again as previous year practical marks will be added.
Internal assessment marks will not be added , your theory marks will be converted into assessment marks.
E.g. if you score 65/80 then your marks will be 65/80*100 = 81
This is the new pattern for internal assessment for improvement exam.
Kindly specify your board in which you want to give improvement exam for specific information.