I'm new to this stuff, so sorry for a trivial question. I bought a RPi kit with RPi 3 B 1.2 for learning purposes. It had GPIO extension board v3.0 . My RPi's GPIO has 40 pins, this extension board has 26 pins. Is it a mistake or is it possible to use this extension board with my RPi somehow? :)
If you can physically fit the board over the first 26 pins on the expansion header it should be fine. Early Pis only had 26 pins.
See https://pinout.xyz/ for a diagram.
Pin 1 is nearest the end of the board with the SD card slot.