public String selectedType {get; set;}
 public String tskId{get;set;}
 public List<task> lstask{get;set;}
 public Search1(){

     lstask=[select AccountId,priority,status,ActivityDate from task];
 }
 public List<SelectOption> getTask()
 {
    List<SelectOption> taskNames = new List<SelectOption>();

    taskNames.add(new SelectOption('','--------Select-----'));
    taskNames.add(new SelectOption('today','Today'));
    taskNames.add(new SelectOption('THIS_WEEK','This Week'));
    taskNames.add(new SelectOption('THIS_MONTH','This Month'));

    return taskNames;
 }
 public void show(){
        /* lstask=[select Id,AccountId,priority,status,ActivityDate from task where createddate < selectedType]; */

        if(selectedType == 'today')
        {
            lstask= [select Id,AccountId,priority,status,ActivityDate from task where createddate = today];
        }
        else if(selectedType=='THIS_WEEK')
        {
            lstask= [select Id,AccountId,priority,status,ActivityDate from task where createddate < THIS_WEEK];
        }
        else if(selectedType == 'THIS_MONTH')
        {
            lstask = [select Id,AccountId,priority,status,ActivityDate from task where createddate < THIS_MONTH];
        }

 }

 public void editfields()
 {

     tskid= tskid.substring(0,15);
     lstask =[select Id,AccountId,priority,status,ActivityDate from task where Id =: tskid];


 }

<apex:actionFunction reRender="frm1" name="act" status="process" action="{!editfields}">
      <apex:param assignTo="{!tskid}" name="tid" value=""/>
    </apex:actionFunction>
    <apex:pageBlock >
        <apex:selectList value="{!selectedType}" multiselect="false" size="1">
            <apex:selectOptions value="{!task}"/>
                <apex:actionsupport event="onchange" action="{!show}"/>
        </apex:selectList>


        <apex:pageBlockSection columns="1">
            <apex:pageBlockTable value="{!lstask}" var="t">
                <apex:column headervalue="Action">
                    <apex:outputLink value="" onclick="act('{!t.id}')" >Edit</apex:outputLink>
                    &nbsp;
                </apex:column>
                <apex:column value="{!t.id}" />
                <apex:column value="{!t.priority}"/>
                <apex:column value="{!t.status}"/>
                <apex:column value="{!t.AccountId}"/>
                <apex:column value="{!t.ActivityDate}" />
            </apex:pageBlockTable>
        </apex:pageBlockSection>
    </apex:pageBlock>

</apex:form>