Any body knows how to convert inches dimension(all ready drawn diagram) to mm in autocad 2013

https://knowledge.autodesk.com/support/autocad/learn-explore/caas/sfdcarticles/sfdcarticles/Change-existing-dimensions-from-imperial-to-metric.html

https://knowledge.autodesk.com/support/autocad/learn-explore/caas/sfdcarticles/sfdcarticles/Convert-imperial-unit-drawing-to-metric-units.html

The second option makes the most sense to me. It makes use of the Scale feature.

If you mean you have a drawing in Inch and you want to put it in another drawing that uses millimeter
- open AutoCAD
- type UNITS
- Insertion scale: INCHES
- OK

- type INSERT
- Select the drawing file (with Inches dimesnions)
- select the insertion point from the screen

The drawing will be inserted and scaled correctly.

Hope that is what you want :)