There are so many tools available to do this type of job. Generally all the tools ask for purchase to know full password. You can visit the referred URL to download the software to crack the password of rar files. This tool is available for evaluation purpose with full functions without any time limit. You can use this tool to do your work.
The next one is to crack the file. Many times it will not create good results. Anyway try it. See Dinesh answer for cracking software.
It is difficult TO crack the password of winrar.
But in simple way you can download software of cracking password . you can download it from rarpasswordcracker.com
Best of luck