To cancel a celebration in FIFA 19 you need to press the L1/LB, L2/LT, R1/RB and R2/RT buttons at the same time. This will cancel a celebration and allow you to perform a different one.
This has been implemented in the game as it is easy to press the wrong button in the excitement of having just scored a goal and you may need to think about what celebration you want to use at that particular time.
If you want to check out which other celebrations are available in the game you can see our full guide here.