Moving to Scraps
Simply view the deviation you wish to change and click on edit, next click on 'choose', under the section "Category", then select 'My Scrapbook' ( this will automatically place your piece in your scraps section ) in the category selector, proceed by clicking on 'OK'. Next finish up by clicking on 'Save Changes'.
Moving out of Scraps
Simply view the scrapbook entry and click the 'Edit Deviation' link located next to it. You can then change the section for this submission from the resulting gallery listing.