change job parameter value in Groovy System Script

If I understand correctly, replaceAction should do the trick (there is also addOrReplaceAction):

import hudson.model.ParametersAction
import hudson.model.ParameterValue
import hudson.model.StringParameterValue
def newMailParameter = new StringParameterValue('MAIL_PARAM', '...')
build.replaceAction(new ParametersAction(newMailParameter))

Edit : if you get error "current build does not have any parameter" then please try "build.addOrReplaceAction" in place of "build.replaceAction".

modify from setBuildParameters: http://jenkins-ci.361315.n4.nabble.com/Modifying-a-builds-parameters-in-a-system-Groovy-script-td4636966.html

def addOrReplaceParamValue = { String name,String value ->
    def build = currentBuild.getRawBuild();
    def npl = new ArrayList<StringParameterValue>()
    def pv = new hudson.model.StringParameterValue(name,value);
    npl.add(pv);
    def newPa = null
    def oldPa = build.getAction(hudson.model.ParametersAction.class)
    if (oldPa != null) {
        build.actions.remove(oldPa)
        newPa = oldPa.createUpdated(npl)
    } else {
        newPa = new hudson.model.ParametersAction(npl)
    }
    build.actions.add(newPa);       
};  
addOrReplaceParamValue("P1","p1");