With regards to making changes to the flight booking, there are a set of steps to be followed using which the required changes can be easily made to the reservation. The steps to be followed are as follows:

Visit www.klm.com, the official website of KLM Airlines.
Go to the My Trips section and recoup the booking you have using the booking reference number and the last name of the passenger.
On the summary page, click on the menu icon list and select the change option.
This shall guide you through the required process to be followed.
Once done, you shall receive a confirmation email from the airline to the registered email address.

How much is the change fee for KLM?

KLM Flight change fee is different for different bookings made. This shall mostly depend on the fare rules of the reservation. However, the average cost shall vary between $30 to $70. Thus, you can refer to the customer executive of the airline so that you can get the changes and modifications done.

How do I contact KLM for a flight Change?

You may get through to change the flight booked through the call process. To contact the airline, you may directly dial 1 800 319 4033 and then follow through the given IVR instructions so that you can easily connect with the airline to facilitate the change process. The call process is as follows:

Dial the number, and choose the language required.
You shall get a set of IVR steps, listen to them, and choose one.
Soon, you can connect with the airline's team and help find a solution.
Once connected, you can explain the requirements for their aid.
 
Note- The changes have to undergo the policies and guidelines. Thus, refer to it from the official website and proceed.