This is bit silly question or I don't know how exactly to post.
I have a circuit with 20 microstrain resolution.
I want know how much force I have to apply on strain gauge so that it provide 20 microstrain change.

Obviously that change will come with change in resistance. That math I can calculate.

But I want to know how to force applied to strain

 

You have not provided enough information. 20µ-strain means a 20ppm deformation from the original. You have to know the properties of the device being deformed in order to relate the ppm deformation into an absolute force.

 

Wayne is right, you are short of information. The Force a strain gauge measures is in (applied to) the deforming body it is attached to, not in the stain gauge itself.

Do you understand how strain gauges work?

Incidentally 'resolution' has nothing to do with your question.
Resolution is always the smallest difference in the measured quantity (strain in your case) your equipment can distinguish.

So a voltmeter with 0.01 volt resolution can distinguish between 5.00 volts and 5.01 volts, but not between 5.006 volts and 5.010 volts and so on.