[SalesForce] Upsert from a trigger?

I want to update records before insert if there is a match on a lookupkey rather than inserting a new record. I have viewed the related question but do not understand how to get the trigger list to update rather than insert a new record? I appreciate everyone taking the time to review, and help a new coder out.

Trigger:

trigger TINCheck on Account (before insert) {
    dupeCheck.checkforDupes(trigger.New);
}

Class:

//A Utility class to process Account records 
public class dupeCheck {

    //the AccountTINTrigger list is getting passed in by our trigger; this should match trigger.new
    public static void checkforDupes(List<Account> itemsFromTrigger){

        //list variable for our query to use as a limit/bind variable
        List<String> namesFromTrigger = new List<String>();
        //populate that list variable
        for(Account acct: itemsFromTrigger){
            namesFromTrigger.add(acct.LLC_BI__lookupKey__c);
        }

        //query for existing items, but only if they have the same TIN (WHERE cond)
        List<Account> existingRelationship = ([SELECT id, LLC_BI__lookupKey__c  FROM Account 
            WHERE LLC_BI__lookupKey__c in :namesFromTrigger]);

        List<account> Recordstoupdate = new List <Account>();
        //check to see if there are matches
        if(existingRelationship.size() > 0){
            //this means we have some dupes and we need to identify which records are dupes
            //loop through Accounts

            for (Account currentRelationship: existingRelationship){
                for(Account newRelationship: itemsFromTrigger){
                    if(currentRelationship.LLC_BI__lookupKey__c == newRelationship.LLC_BI__lookupKey__c) {
                        newRelationship.Id = currentRelationship.Id;}
//What else is needed here to update the itemsFromTrigger so that records are upserted rather than inserted?
                }
            }
        } 
    }
}