You can boot into safemode and then replace it in CMD (copy shell32.dll C:\windows\system32\shell32.dll) and that'll replace it since it wont be in use.
Or you can go start > run > cmd > and type in tasklist. Then find the process shell32.dll, and type in taskkill /im shell32.dll /t /f and that should terminate the process and any other ones it started. "shell32.dll" would need to be the process name otherwise it wont end. Then replace it with CMD.