Hi there,

am a technical guy, so I will attempt a more technical answer because I don't think there is a way you can do this without code.

Whenever a lead is converted, an opporrtunity will be inserted alongside an account or contact. If you have logic that you want executed in a trigger, it has to be on insert. If it has to be on update for whatever reason (e.g. reparenting), then you will have to retrieve the inserted opportunity and update it. Note that you have to check this option under Customize>Leads>Settings for all Apex triggers to run on lead conversion "Require Validation for Converted Leads"

That said, in Apex code you will have access to the Ids of the created account, opportunity and contact. The basic idea is something like this,

Database.LeadConvert dlc = new Database.LeadConvert();
Database.LeadConvertResult dlcr = Database.convertLead(lc);
Id opportunityId = dlr.getOpportunityId();
Opportunity insertedOpp = [SELECT Id, Name FROM Opportunity WHERE Id=:opportunityId];

Do something with the opportunity or just run an update on it if you logic is already in an update trigger.

try {
    update insertedOpp;
} catch(Exception e){
    System.debug('An error occured while updating an opportunity after lead conversion : ' + e.getMessage());
}

Hope this was helpful. Fire back in case of questions