I am new to apex coding but thought I understood most of what I need to do.  I have created a vf page that displays some fields amd updates some fields.  When I execute it, I get the following error: "DML not currently allowed" due to the DML "update" statement.  This has me puzzeled as it works from the Anonymous Window.  I would appreciate any help on this.  Following is the class and VF Page. Thanks.

<apex:page controller="Test2">
    <apex:form >
    <apex:pageblock title="Greetings {!$User.FirstName}">
        <apex:pageBlockTable value="{!Clt}" var="c">
            <apex:column value="{!c.Name}"/>
            <apex:column value="{!c.Gender__c}"/>
            <apex:column value="{!c.Age__c}"/>
             <apex:column value="{!c.SDLM__c}"/>
 
        </apex:pageBlockTable>
    </apex:pageblock>
    </apex:form>                                           
</apex:page>


public with sharing class Test2 {
        Public Test2() {
        }
        Public List <Client__c> clt;
        Integer sdlm=1;
         
        Public List <Client__c> getclt (){
            clt = [select Client__c.Name,Client__c.Age__c,
                   Client__c.Gender__c,Client__c.SDLM__c
            From Client__c
                   Order by Client__c.Name DESC];   
     
            For(Client__c c: clt){
                c.SDLM__c = 1;
            }   
                     
            Update clt;
            return clt;
            }
 }