How to change this smartlock battery

I have to change the battery of our Nuki Smartlock. It was a thing of my husband, but he passed away a few weeks ago. I can’t find the instructies manual and the pictures online are not from the same battery. I hope someone van help me.

I got the battery holder out of the smartlock, but can’t get the 4 batteries oud of the holder. Ther is some kind of cap on the batteries that I can’t get of. (See picture)
What should I do next?

Hello! Very sorry for your lost! This on the picture is a accupack, you can’t change the batteries, but at the bottom is a usb-c port, and here you can plug in just a phone charger (or similar) and load it! You can also for loading, place the accupack in the nuki and plug in the usb-c charger, because then you can see the accu-load in percent, and watch it, but also the charging will stop automatically when the accupack is full! Hope this helps!

Thank you both for the help! I did it and the battery is full!