On what Version is your DSi please ?
Do you have some issues ?
If you really want to update,the easiest Way is to copy the UNLANCH.DSI on the Root of your SD Card and power on your DSi while holding BUTTON A.You get into Unlaunch.Then select UNLAUNCH.DSI in the List and start it.Select "Install now" on the next screen.
Please be sure that your DSi´s Battery is charged. On the next Boot Up you get into Unlaunch again and you have to set your Boot Up Apps again:
-NO BUTTON - choose TWiLight Menu++ from the List (sdmc:/BOOT.ND5) to get into TWiLightMenu
The other Buttons as you like please.
!!! DO THIS ON YOUR OWN RESPONSIBILITY AND YOUR OWN RISK !!!