How can I replace all value fields in xml file having perticular key value using sed command in unix

How can I replace all value fields in xml file having perticular key value using sed command in unix? Example, I have file.xml with me and I want to change the value to 256m which has key= "-Xmx"

Example

<entry key="-Xmx" value="512m" />

I want to replace value as "256m" for any entry having key as "-Xmx"