If you have a reservation with KLM, you can choose the following way to select your seat.

Select KLM seat online: -

You can make your seat selection while making a reservation and later before departure.
During reservation, you can select your seat before the payment; if it doesn’t happen, you can go for the second option after reservation.
To select your seat after reservation, you can go to the MY trip option, where you can retrieve your booing after entering booking ref and last name.
* Once you find your trip details, you can go to the change flight options, and there you can follow the instructions on the screen to select your seat.
You can also contact the KLM customer service to make a seat selection by speaking to the representative.
So using the above-given methods, you can select your seat anytime on KLM airline. Besides, if you want to upgrade it later to an extra comfortable seat, you can pay the KLM seat upgrade price and get your seat upgrade done. You can upgrade your seat online and by calling the reservation department.