Hello ​@Chi Chung LO 

You have only one direct train from Milan to Lucerne every day, you can change your reservation only for this train on another day.

I suggest to cancel this reservation, if it is possible, and to take a regional train from Milan to Lugano and an Intercity from Lugano to Lucerne, both without reservations (see the green connections).

If I give up the original train, can I join the other manually, such as 13:10 ?

What do you mean by "join the other manually”?

I mean join the next train, or others also need to reservation required train.

Sorry about my poor English.

Such as the 13:10 scheduled train. Also is seat reservation required. Can I join that?

​@Chi Chung LO 

If you take the 13:10 train, you must make a new reservation for this train, but only from Milan to Arth-Goldau, because you must change at this station to go to Lucerne (you see in the timetable: seat reservation required).

I would take the 13:43 train, you don’t need any reservation, you can scan only your Eurail Pass to open the gates in Milan station and show it if a control occurs in the train (you see in the timetable: no seat reservation required).

Solution 1 : If I take the 13:10 train, I need to pay again the reservation fee

Solution 2 :  take the 13:43 train, no additional fee but I waste the original reservation fee

its my understanding

Reservations for the EC Italy-Switzerland booked via Eurail are 80% refundable. And you always loose Eurail's €2 booking fee. That means a refund of 80% of €13 (€10.40) per seat should be possible.

Also see:

https://www.eurail.com/en/eurail-passes/ordering-info/refund-exchange-policy/exchange-and-refund-conditions-for-reservations