vincenzo@home:~$ cat provadoc
<upower>
        <ciao></ciao>
        <coia></coap>
        <Theme>my</Theme>
        <Theme>pepe</Theme>
        <Theme>ciccio</Theme>
</upower>
vincenzo@home:~$ sed -e's/<Theme>/<miotema>/1' provadoc
<upower>
        <ciao></ciao>
        <coia></coap>
        <miotema>my</Theme>
        <miotema>pepe</Theme>
        <miotema>ciccio</Theme>
</upower>