DosReplaceModule replaces an active DLL or EXE by caching the contents of the old module and then copying a new file over the old module's file.

#include <os2.h>

ULONG     pszOldModule;
ULONG     pszNewModule;
ULONG     pszBackupModule;
APIRET    rc;

rc = DosReplaceModule(pszOldModule, pszNewModule,
       pszBackupModule);