how to update the xml file using xmlstarlet

You did not show your input document, but I assume it is the following, taken from the xmlstarlet documentation:

<xml>  <table>    <rec id="1">      <numField>123</numField>      <stringField>String Value</stringField>    </rec>    <rec id="2">      <numField>346</numField>      <stringField>Text Value</stringField>    </rec>    <rec id="3">      <numField>-23</numField>      <stringField>stringValue</stringField>    </rec>  </table></xml>

xmlstarlet modifies the file, but the result is sent to standard output, not saved in the original file. Use another option --inplace to modify the file in place:

$ xml ed --inplace -u "/xml/table/rec[@id='3']/@id" -v 5 rec.xml

Then:

$ cat rec.xml<?xml version="1.0"?><xml>  <table>    <rec id="1">      <numField>123</numField>      <stringField>String Value</stringField>    </rec>    <rec id="2">      <numField>346</numField>      <stringField>Text Value</stringField>    </rec>    <rec id="5">      <numField>-23</numField>      <stringField>stringValue</stringField>    </rec>  </table></xml>

By the way, this question seems to ask something very similar to this question.

EDIT: As suggested by @npostavs, this option is listed in the edit help:

$ xml edit --help...-L (or --inplace)   - edit file inplace...