You can do that with curl like that:

curl -k -d awesome -u stef:$pass -X PUT

In this example:

* the resource to update is 'run/ad647b52-457b-4185-bf2d-eba4e35c2e1f/wp.1:mysql_password'

* it is composed of the run: run/ad647b52-457b-4185-bf2d-eba4e35c2e1f

* and the fully qualified runtime parameter: 'wp.1:mysql_password'

* the new value is 'awesome'

* the SlipStream user name is 'stef'

* the Slipstream password is in environment variable 'pass'

Should you need to update a resource in 'abort' state, you must add ignoreabort=true to your request:

curl -k -d awesome2 -u stef:$pass -X PUT