Add a goto with no condition after all your other if commands. If you want the batch to exit after running the start command as well, you can do something like this:
:top
set /p udefine=
echo.
if %udefine%==1 start www.google.com&exit
if %udefine%==2 start www.youtube.com&exit
if %udefine%==3 start www.wikipedia.com&exit
if %udefine%==e goto exit
echo Unrecognized Entry.
goto top