It's very easy.

First, have a look in the Settings menu and see what system you have
The software version displays on the top right of the Settings screen
You may have a Mk2 or a Mk3

The last SW for a Mk2 is 2-1/81
The last SW for a Mk3 is 3-1/63

Check what you have, if you don't have the latest version, then an update is worth while.

From around 2004 the Mk4 appeared, which supports 3D perspective view, and is MUCH MUCH faster than the Mk3 or Mk2.

The current software version for a Mk4 is 4-1/100 (which displays as 4-1/00)

A Mk4 is a straight plug-and-play swap for the Mk3 - and for the later versions of the Mk2.

If you like keeping the car original, then consider fitting a Mk4, and even upgrading from a 4:3 narrow-screen monitor to the 16:9 widescreen monitor (if not already fitted).

The Mk4 has lots of POIs on the DVD maps (which can cover an entire continent) and also 3D view. It is different to a TomTOm, and is also much more accurate then a TomTOm, especially if you drive through lots of underpasses and tunnels (like I do).
The better accuracy in the BMW nav is due to the BMW using the wheel speed sensors and a gyro compass when no GPS signal is present. Makes a whopping difference.

TomTom is nice in that the maps are cheaper and are updated every 3 months, but it is also a decade newer than the BMW system... however nothing beats a fully integrated built-in nav (imho).