Your post is not clear...
You have a BKK -> TLV which is a paid ticket or a bonus ticket?
If its a paid ticket then, depends on your class on
S,Y, M, I, C you can do advanced upgrade which is 700 points and you should
do it few days prior to your flight (can be done i think up to 1 day before the flight) should be around 700 points for you segment)
If you wish an upgrade on the airport (Last Minute) it will be based on availbility and a priority list and will be around 100 points.

You cannot use vouchers at the airport, unless your agent was very bad and issued you an upgrade without putting the voucher number, then you will be asked at the airport for the voucher...

If you are a TPL you can upgrade from any class for 1.5 points (so 1050 points from Q class to C for example).