Composer Downgrade php version 7.1 to 7.0 on windows

I am working on Laravel project where one of its packages require PHP 7.0 version. Currently, I have installed latest version but now I want to downgrade it to 7.0. How should I do this? Is it possible with command line or do I need to change PHP files manually? I tried to find this topic on google also on StackOverflow but can't find any solution for this problem

I found one solution here but unable to implement that

here is the link I tried

This is the error I get with 7.1 version

Function mcrypt_module_open() is deprecated

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