I am attempting to start this Apex class with Process Builder, triggered by a field change. I've never done this before and I'm unsure what invoke code needs to be added in order to accomplish this. I tried adding @InvocableMethod but I'm getting an error telling me this :

Compile Error: Unsupported parameter type String. 
Valid invocable parameters must be List types like List<T> 
where T is a supported type at line 26 column 27

It is a class which is currently being invoked with a Lightning Component button in a record.

public class UpdateContactPCRController {

    @InvocableMethod
    public static String  updateContact(String recordId){        
       
        String contactId = '';       
        
        List<Contact> contactToBeupdate = new List<Contact>();        
        
        if(String.isNotBlank(recordId)){            
            
            List<Program_Contact_Role__c> programContacList = [SELECT Id,Contact__c,Program_Name__c FROM Program_Contact_Role__c WHERE Id =:recordId AND Contact__c != null];
            contactId = programContacList[0].Contact__c;
            
            if(String.isNotBlank(contactId)){
                
                contactToBeupdate = [Select Id,Pardot_Action_Trigger__c,PCR_Register_Button_Link__c,PCR_URL_for_UI__c FROM Contact Where Id =: contactId Limit 1];                
                
                contactToBeupdate[0].Program_Contact_Role_Id__c = recordId;
            }
                        List<Program_Communication_Recipients__c> programCommunicationRecs = [Select Id,Name,
                                  Program_Communication__r.Program__r.Buyer__r.Name,Receipient__c,                
                                              From Program_Communication_Recipients__c Where 
                                                           Program_Communication__r.Program__c =: programContacList[0].Program_Name__c AND 
                                                           Receipient__c =: programContacList[0].Id];                                           
                                
                
                    contactToBeupdate[0].PCR_Welcome_Message__c = String.valueOf(programCommunicationRecs[0].Program_Communication__r.Welcome_Message__c);                                                    

        if(contactToBeupdate.size() > 0){
            update contactToBeupdate;  
        }
        
        return 'Updated Successfully';
    }
}