How can I replace all value fields in an XML file having perticular key value using sed or any other command from Unix?

Example, I have `file.xml` with me and I want to change the value to `256m` which has `key= "-Xmx"` 

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

```

Example:

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

```

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