<apex:pageBlockTable id="thetable" title="Invoice line item" var="acc" value="{!contactList}">
                       <apex:column headerValue="S.No" style="width:20px; text-align:center;" headerClass="centertext">
                         <apex:inputField value="{!acc.Row_Num__c}" id="getindex" style="width:25px" />
                        </apex:column>
                        <apex:column headerValue="Nicomatic Part No"><apex:inputField value="{!acc.Name}" required="true" /></apex:column>
                        <apex:column headerValue="Description"><apex:inputField value="{!acc.Description__c}" style="height:30px" /></apex:column>
                        <apex:column headervalue="Client Part No"><apex:inputField value="{!acc.Client_Part_No__c}" style="width:150px" /></apex:column>
                        <apex:column headerValue="HSN"><apex:inputField value="{!acc.HSN__c}" style="width:60px"/></apex:column>
                        <apex:column headervalue="Quantity">
                         <apex:inputField value="{!acc.Quantity__c}" style="width:60px">
                         <apex:param name="cal" value="{!op2}"/>
                         <apex:actionSupport event="onchange" reRender="tot"/>
                         </apex:inputField>
                         <apex:variable var="op1" value="{!acc.Quantity__c}"/>
                        </apex:column>
                        <apex:column headerValue="Rate(Per Item)">
                          <apex:inputField value="{!acc.Rate_Per_Item__c}" style="width:80px">
                          <apex:param name="cal" value="{!op2}"/>
                           <apex:actionSupport event="onchange" reRender="tot"/>
                          </apex:inputField>
                          <apex:variable var="op2" value="{!acc.Rate_Per_Item__c}"/>
                        </apex:column>
                        <apex:column headerValue="total">
                        <apex:outputpanel id="tot">
                        <apex:outputText value="{!(acc.Quantity__c*acc.Rate_Per_Item__c)}">
                        <apex:actionSupport event="onchange" rerender="to"/>
                         </apex:outputText>
                         </apex:outputPanel>
                        </apex:column>
                        <apex:column headerValue="Discount"><apex:inputField value="{!acc.Discount__c}" style="width:60px">
                        <apex:actionSupport event="onchange" rerender="to"/>
                         </apex:inputField>
                        </apex:column>
                        <apex:column headerValue="Taxable Value">
                        <apex:outputText id="to" value="{!((acc.Quantity__c*acc.Rate_Per_Item__c)-acc.Discount__c)}" />
                       </apex:column>

                        <apex:column headerValue="Action" >
                         <apex:commandButton value="Delete" onmouseout="insert_numbers()" action="{!deleteRow}" reRender="pb">
                          <apex:param name="rowIndex" value="{!rowNumber}"/>
                         </apex:commandButton>
                         <apex:variable var="rowNumber" value="{!rowNumber+1}"/>
                       </apex:column>
                        <apex:column HeaderValue="Row Up">
                        <apex:commandButton onClick="insert_numbers()"
                          rerender="mytable" action="{!swap}"  value="Up" >&#8679;
                           <apex:param name="RowIndex" assignTo="{!rowNumber}" value="{!rowNumber}" />

                        </apex:commandbutton>
                    </apex:column>
                    <apex:column HeaderValue="Row Down">
                        <apex:commandButton onClick="insert_numbers()"
                            rerender="mytable" action="{!swap}" value="Down" >&#8681;
                            <apex:param name="RowIndex" assignTo="{!rowNumber}" value="{!rowNumber}" />

                        </apex:commandbutton>
                    </apex:column>
         </apex:pageBlockTable>

Public Void Swap(){
    //Here this **"contactList"** is normal list that I'm passing information to //VF page to pageblock table
//I don't know what to do now for Swapping
                    For(Invoice__c li:contactList){

                        If(li.name=='656555'){

                            li=li[0];
                        }
                    }

            }