How to reset filter values in react admin framework?

There is a setFilters prop in your filter component, you can use it:

export const PostsFilter = (props) => (
  <div>
    <Filter {...props}>
      <TextInput label='Post ID' source='id' alwaysOn />
      <TextInput label='User ID' source='user_id' alwaysOn />
    </Filter>
    <Button onClick={() => props.setFilters({
          id: '',
          user_id: ''
     })}>Clear fields</Button>
  <div>
);