Hi, I've tried to wrote the concept down .... I didn' check the syntaxt so it could be the case that some misspells are in the code ... but you will see how it works!

trigger SetContractdetailsOnContact on Opportunity (after insert, after update) { 
     // extract the contact id's fom the opportunities and store them in a list 
     List contactIDs = new List(); 
     for (Opportunity opp : trigger.new){ 
          if(opp.YOURCONTACTFIELD__c != null){ 
               contactIDs.add(opp.YOURCONTACTFIELD__c); 
           } 
     } // now query the contacts and put them in a Map 
   
     Map contactMap = new Map(); 
     for(Contact contact : [SELECT ID, ContractLength__c, ContractServiceDate__c from Contact Where id in :contactIDs]){ 
           contactMap.put(contact.id, contact); 
     } 
     // iterate over the opportunities and update the contact records 
     for(Opportunity opp : trigger.new){ 
         // only proceed if the CONTACTFIELD is not null - else skip the opportunity 
         if(opp.YOURCONTACTFIELD__c != null){ 
              // get the contact from the map 
              Contact c = contactMap.get(opp.YOURCONTACTFIELD__c); 
              // double check if the contact is really there - if not skip the record 
              if(c != null){ 
                    // check if the trigger is in the insert mode 
                    if(trigger.isInsert){ 
                            // if contractLength is set update the corresponding field on the contact record 
                            if(opp.ContractLength__c != null){ 
                                 c.ContractLength__c = opp.ContractLength__c; 
                            } 
                            // if ContractServiceDate is set update the corresponding field on the contact record 
                            if(opp.ContractServiceDate__c != null){ 
                                 c.ContractServiceDate__c = opp.ContractServiceDate__c; 
                            } 
                     }else if(trigger.isUpdate){ 
                            // if the trigger is in the context of updating oppty records 
                            // if contractLength is set AND changed update the corresponding field on the contact record 
                            if(opp.ContractLength__c != null && trigger.oldMap.get(opp.ID).ContractLength__c != opp.ContractLength__c){ 
                                  c.ContractLength__c = opp.ContractLength__c; 
                            } 
                            // if ContractServiceDate is set AND changed update the corresponding field on the contact record 
                            if(opp.ContractServiceDate__c != null && trigger.oldMap.get(opp.ID).ContractServiceDate__c != opp.ContractServiceDate__c){ 
                                  c.ContractServiceDate__c = opp.ContractServiceDate__c; 
                            } 
                      } 
               } 
         } 
     } 
     upsert contactMap.values(); 
}

  If this answer your question pls mark the entry as resolved!