i'm trying to update the Account in opportunity for account field null can anyone help me on this

// You can try this code
trigger AccountCreationFromOpportunity on Opportunity (before update) {
// Create a Map op Opportunity Id to Account
//  It would be trigger.New  sicnce you would be updating in Update call. Trigger.old is readonly
Map<Id,Account> mapOppIdToAccount = new Map<Id,Account>();
for(Opportunity o :trigger.New){
        if(o.AccountId == NULL){
        Account acct = new Account();
        acct.Name = 'New Account';
        // Add the opp Id as key and  new account  as values in the map
        mapOppIdToAccount.put(o.id,acct);
        }
         
     }
     // If the new Account List/Map is not empty then Insert accounts 
     if(!mapOppIdToAccount.IsEmpty()){
        Database.Insert(mapOppIdToAccount.Values()); 
     }
     // Execute the for loop on trigger.new again
    for(Opportunity opp :trigger.New){
        if(opp.AccountId == NULL){
        //Checking if the  New account is present or not
        if(!mapOppIdToAccount.IsEmpty() && mapOppIdToAccount.ContainsKey(opp.Id)){
        //Update the AccountId field on Opportunity record by fetching account Id from the Map that we inserted
        opp.AccountId = mapOppIdToAccount.get(opp.Id).Id;
        }
    } 
    }
}

This is the bulkified version of the code

trigger OpportunityTrigger on Opportunity(before update){
    if(Trigger.isbefore )
    {
        if(Trigger.isUpdate ) {
            List<Opportunity> opps = new List<Opportunity>();
            for(Opportunity o : Trigger.new){
                if(o.AccountId == NULL){
                    opps.add(o);      
                }
            }
            Account acc = new Account();
            acc.Name = 'Test123';
            insert acc;
            for(Opportunity o : opps){
                o.AccountId = acc.Id;
                
            }
            
        }
    }
}

It is not recommended to create an Opportunity without an Account basically an Orphan Opportunity. If you want an Orphan Opportunity to get a Parent(account) this code above does that for you.

But the thing is it will create a new Account record every time the opportunity trigger is fired.

You can change that to a already created record of Account object and refer the Account Id in opportunity to the Id of that account record.

Please, try to give as much information with the question to make it understandable in no time. And mark the answer useful if it gives what you need. Cheers :)