See https://developer.yahoo.com/fantasysports/guide/ for details

roster_put_nfl_example.xml

<?xml version="1.0"?>
<fantasy_content>
  <roster>
    <coverage_type>week</coverage_type>
    <week>13</week>

    <players>
      <player>
        <player_key>242.p.8332</player_key>
        <position>WR</position>
      </player>
      <player>
        <player_key>242.p.1423</player_key>
        <position>BN</position>
      </player>
    </players>
  </roster>
</fantasy_content>

roster_put_other_example.xml

<?xml version="1.0"?>
<fantasy_content>
  <roster>
    <coverage_type>date</coverage_type>
    <date>2011-05-01</date>

    <players>
      <player>
        <player_key>253.p.8332</player_key>
        <position>1B</position>
      </player>
      <player>
        <player_key>253.p.1423</player_key>
        <position>BN</position>
      </player>
    </players>
  </roster>
</fantasy_content>