Laravel change input value

If you're looking to do this in Laravel 5, you can use the merge() method from the Request class:

class SomeController extends Controller
{
    public function someAction( Request $request ) {

        // Split a bunch of email addresses
        // submitted from a textarea form input
        // into an array, and replace the input email
        // with this array, instead of the original string.
        if ( !empty( $request->input( 'emails' ) ) ) {

            $emails = $request->input( 'emails' );
            $emails = preg_replace( '/\s+/m', ',', $emails );
            $emails = explode( ',', $emails );

            // THIS IS KEY!
            // Replacing the old input string with
            // with an array of emails.
            $request->merge( array( 'emails' => $emails ) );
        }

        // Some default validation rules.
        $rules = array();

        // Create validator object.
        $validator = Validator::make( $request->all(), $rules );

        // Validation rules for each email in the array.
        $validator->each( 'emails', ['required', 'email', 'min: 6', 'max: 254'] );

        if ( $validator->fails() ) {
            return back()->withErrors($validator)->withInput();
        } else {
            // Input validated successfully, proceed further.
        }
    }
}

You can use Input::merge() to replace single items.

Input::merge(['inputname' => 'new value']);

Or use Input::replace() to replace the entire input array.

Input::replace(['inputname' => 'new value']);

Here's a link to the documentation