As far as I know, they are all pressed in. To get them out, you need to push them out. This is rarely needed. Good cartridges should last a very long time. But, if you buy a cheap hub and want to have it run very smooth, you can buy very good cartridges for a little bit of money and replace the cheaper ones.
If you want to give it a try, look at
this blog, It should give you the information you need to know.