Polymer 1.0, how to pass down attributes to another component

I don't know what the term would be in Polymer but I want to pass data from one polymer element to another element that it is using inside it.

    <dom-module id="bw-boat-form">      
      <template>
          <paper-material class="material-container" elevation="1">
            <h2></h2>
            <form is="iron-form">
                  <bw-image-upload image$="" ></bw-image-upload>                  
            </form>
          </paper-material>
       </template>

  <script>
        Polymer({
          is: 'bw-boat-form',
          properties: {
              data: {
                type: Object
              },
              save: {
                type: Boolean
              },
              update: {
                type: Boolean,
                value: false
              }
          }
        });
  </script>
</dom-module>

And the image upload element

 <dom-module is="bw-image-upload">     
  <template>
      <iron-image class="image" preload placeholder="/images/icons/placeholder.jpg" sizing="cover"></iron-image>
      <vaadin-upload
                      target="/images/upload"
                      max-files="1"
                      max-file-size="200000"
                      accept="image/*"
                      upload-success="uploadResponseHandler"
                      file-reject="errorHandler"
      >
      </vaadin-upload>
  </template>
  <script>
  Polymer({
            is: 'bw-image-upload',
            ready: function() {
                console.log(image);
            },
            uploadResponseHandler: function() {

            },
            errorHandler: function() {

            }
          })
  </script>
</dom-module>

I want to pass down the value of the from the first element into the second element so that I can change the image of the placeholder when needed. How can I achieve this ?