Are you confuse to upgrade your seats on Singapore flights? Take a look of this presentation.

1. Singapore Airlines Business/First Class Upgrade
Below are three easy ways to upgrade your Singapore Airlines
ticket to premium economy, business class, or first class. The
Singapore Airlines policy allows ticket holders to upgrade to
Business Class or Premium Economy if there is space available.
Members of Velocity can use their points for upgrades to premium
economy or award seats in Business class on all Singapore Airlines
routes excluding those to and from China and Hong Kong. In
addition to the already reduced prices of Lite fares, award tickets
can be upgraded to the next cabin level higher using KrisFlyer miles
(or two cabins from Economy class to Business class on planes that
have Premium Economy). As you can see, the value of using
KrisFlyer miles to guarantee a business class seat after purchasing
a full-price Economy ticket on an upgradable fares code is reduced
in both scenarios, with an estimated value of around 1.3 cents per
mile after factoring in the miles you would have earned for a basic
ticket.
From Singapore, you can fly to London or Paris in business class
for 65,000 KrisFlyer miles one way in flexible fares or 80,000 miles
in economy standard tickets. This is somewhat less than the 85,000
miles required for a direct Business class award. To be eligible for
an award upgrade from Economy Standard, your Singapore Airlines
or SilkAir ticket must be booked as an M, H, or W fare. You can use
miles or points to upgrade your ticket either at the time of booking
or after purchasing an economy ticket through the airline's online
reservation system.

2. Buying a ticket in business class and Singapore Airlines
Upgrade. First or business class is the most cost-effective option.
If you want to upgrade to business or first class, you can try your
luck at an auction with several airlines. Some upgrades are
mixed-itinerary, meaning that if your flight has a layover, you may
be in economy on one leg and business or first on the next. This
can be a bad deal if the leg on which you are upgraded is shorter.
Despite the preceding, no refunds will be given for the cost of the
upgrade or the miles used to purchase it if the passenger reeboks
into a seat of the same or higher class than the upgraded ticket on
the same aircraft or on a different flight. For the avoidance of doubt,
the purchase of an upgraded seat does not result in the
accumulation of KrisFlyer miles, elite miles, PPS values, or miles on
any other KrisFlyer Partner Airlines. If you paid for a premium
economy upgrade and then got bumped up to business class, that's
not allowed because mySQupgrade only allows one upgrade per
segment (however, as shown in the above screenshot, an economy
passenger can purchase an immediate Business Class Upgrade).
Another option is to just pay the fare difference to upgrade to any
fare class. However, not all tickets can be upgraded, so don't
expect that if you buy a cheap fare.