Yes, any passenger who has purchased a plane ticket can easily upgrade it if they wish. Ticket upgrades can be done at any time between the time of reservations and before the closing of flight check-in and can also be done in a number of ways.

How do I make my flight upgrade?

Travelers who have purchased a flight ticket and are willing to upgrade then they can easily upgrade plane ticket by online and offline approaches and here are both explained. 

Online Upgrade Process: You will find the online method of upgrading tickets more convenient because you will be doing the process by yourself at your own pace. Here, you just need to follow a simple process as given.

• Go to your airline's official website, and then choose My Trip tab.
• You can use the ticket credentials to find your flight and then enter into the manage my booking page. 
• Once you reach it, find the upgrade option, make upgrades, and select your seat.
• Pay for the upgrade as applicable and then enjoy the benefits of it.

Offline Upgrade Process: Those who are unable to make their ticket upgrade from the online process can easily use the offline approach, where they have basically two options, and in both, they will interact with the airline's representative over the call or at the airport. Use the airline's customer service telephone number and follow the calling process to upgrade the ticket through the call, or you can directly visit the airport ticketing counter and talk to the floor coordinator. The assistant will upgrade your ticket by taking your fare difference.

How much did the ticket upgrade cost me?

Normally, when a person upgrades their flight ticket, they will be paying a sum of the service fee, fare difference, and applicable tax. The Cost of upgrading a flight ticket may vary from airline to airline and also depends on various factors, including the type of fare, the class in which you're upgrading your flight ticket, the route you are flying, the number of passengers on the ticket, the time of upgrading, and others. In most of the cases, it starts at $100.