將如下程式碼追加到NSIS指令碼的尾部即可。

Var UNINSTALL_PROG
Var OLD_VER
Var OLD_PATH

Function .onInit
  ClearErrors
  ReadRegStr $UNINSTALL_PROG ${PRODUCT_UNINST_ROOT_KEY} ${PRODUCT_UNINST_KEY} "UninstallString"
  IfErrors  done
  
  ReadRegStr $OLD_VER ${PRODUCT_UNINST_ROOT_KEY} ${PRODUCT_UNINST_KEY} "DisplayVersion"
  MessageBox MB_YESNOCANCEL|MB_ICONQUESTION \
    "檢測到本機已經安裝了 ${PRODUCT_NAME} $OLD_VER。\
    $\n$\n是否先解除安裝已安裝的版本？" \
      /SD IDYES \
      IDYES uninstall \
      IDNO done
  Abort
  
uninstall:
  StrCpy $OLD_PATH $UNINSTALL_PROG -10


  ExecWait '"$UNINSTALL_PROG" /S _?=$OLD_PATH' $0
  DetailPrint "uninst.exe returned $0"
  Delete "$UNINSTALL_PROG"
  RMDir $OLD_PATH


done:
FunctionEnd