I had to open one once when I pulled on the DC cable too much and was worried that I'd damaged it. You can't open them without cracking the plastic and then damaging the inner metal case as you pry it open. Also, the pins of the AC socket are attached to the plastic, so I ended up having to solder in the mains cable as the socket was completely broken. After all that, the DC cable was fine anyway. It's now held together with hot melt glue, electrical tape and cable ties. Luckily it was my spare one anyway.
In short, you're better off buying a new one.