How do I convert an IIR filter into a FIR filter in digital signal processing?

Maybe you were asking if there is some kind of design tool allowing to convert an IIR filter into an FIR filter automatically. There is no such a program. IIRs and FIRs have fundamentally different characteristics. 

If you are just interested in designing an FIR which approximates the amplitude response of an IIR,  first of all you need to analyze the amplitude response of the IIR and extract the main parameters (passband and stopband cutoff frequencies, passband ripple, and stopband rejecton). You can design the FIR filter using these parameters as a starting point and varying the number of coefficients. Besides changing the number of coefficients you will have also to tweak the parameters to improve the approximation. You can also try different FIR design methods to see which one allows you to achieve the best result.