XML : xmlstarlet to change/update value

I am trying to change the value to an xml file using xmlstarlet.

Here is the lines of my xml:

  <program xsi:type="server" name="server1">    <log enabled="true" level="6" period="1" periodUnits="DAY" numberRetained="30" haltThreshold="500" reduceThreshold="1024">/var/log/server1</log>    

I am trying to update the level to any other value. as 2 or 3... but first to query the current value.

  xmlstarlet sel -t -c "/programs/program/type="server"/name="server1"/log" file.xml    

this giving me "true".. ?

Many thanks for your hints or response. Al.