Firstly, there's a hole for a locking pin in the connectors. The pin prevents the connectors from sliding apart. If there's a pin in there, you'll need to push it out with a punch.
If there isn't a pin, it's possible the connector was assembled with cyanoacrylate glue. Since the roll pin can sometimes fall out, Anderson recommends glue in applications where this would be a problem. In this case you may not be able to get the connectors apart without damage.
With the roll pin removed, the connectors slide apart. Each side of the connector will have either a groove or a tongue. If one side has a groove, the opposite side will always have a tongue, so you should be able to determine the necessary sliding direction by looking at the connector. An image helps:
If there are wires attached to the connector it might be hard to slide connector apart. If the wire is zipline, it may be possible to separate the wires for a few inches so there's enough flexibility to move the connectors.
If that's not possible, then you may be able to get the contacts out of the housing. There's a tool for doing this. If you don't have or don't care to purchase the tool, then you may be able to improvise something similar with a bent piece of metal.
The contacts are held in place by an internal spring.
You need to get something around and behind the contact to push up the spring. The spring is what retains the contact: by lifting the spring the contact will be free to slide out.
Take care to avoid bending the contact when doing this: if the contact is bent it may no longer make solid contact with the mating connector. Where possible I'd advise discarding the old contacts and attaching new contacts.