VNAV app
 

I think it is better to update the CRZ level on the FMC if you start the approach on higher altitude than the platform height.
any comments?

That's your opinion, and you're fully entitled thereto.

vnav
 

any other way then?

Depending on configuration of VNAV it may assume a 1000'/min descent rate until reaching the platform = new CRZ Altitude.

Be careful with this as the T/D generated is for new CRZ Altitude, thus if not careful you may get high before flying the approach if your aircraft has not yet reached this new altitude.

Therefore I would strongly recommend you not to use this primative method.

However if you have an extended level segment (such as an early descent profile London ATC can give you [descent from CRZ @ FL390 to fly 80nm @ FL250]) then it would be prudent to enter this new level as a new cruise level so ECON CRZ speeds for actual Cost Index , Gross Weight, Wind, OAT and FL can be calculated and flown, rather than the ECON fixed descent speed which is only based on CI.

vnav
 

Thanks but I meant VNAV APPROACH not VNAV during the descent.

Maybe I didn't quite understand your question...
When in the below practical example description of events during a non-precision approach would you propose: Practical example:
Controller vectors you towards final approach course of VOR approach which you have elected to fly as a VNAV Approach.
You select the altitude he clears you to in MCP (you may still be above published platform altitude as he may vector a straight in rather then with a procedural turn for which the platform is used).
You arm LNAV to capture inbound track while VNAV remains engage and will start slowing the aircraft down in good time.
Using the CDA capability VNAV will automatically transition from Idle Decent to Approach Logic, allowing Flap Extension and a continued CDA onto Approach.

Check your FCOM and see when VNAV starts its Approach Mode.
You may find that your actions only make sense if in a descent.
VNAV Approach Logic should be defined in the VNAV literature for your type.

I didn't either, seeing as it was asked so clearly and concisely.

I update every time I get a level off. Seems to work ok.

Could an alternative to the above be to go Selected from that (initial descent) point and continue the approach that way, until say initial approach point (depending on the traffic, ATC, conditions of the day etc. . . )?