No, it is not hot-swappable.  Power down to replace.  Most (not necessarily all) servers that support "hot-swap" are actually "hot-add", meaning you can add memory "hot", but not remove (including replacing).
Hello everyone, a Dell Poweredge 2950 had memory failure. Is memory hot swappable in Dell 2950? Do I have to take the server down before replacing the memory?
Dell PowerEdge 2950 Memory Replacement