Can you please advise the model number, which is on a label located on the lid of a chest freezer or on the left had size of an upright freezer.
To confirm you want to adjust the temperature to -15, its currently set to -33?
If this is the case, this is done by using the temperature regulator knob located to the right of the control panel which displays the current temperature. If you set the dial to around 5 O'clock position it should cool the freezer to around -15, please not it will take time for the temperature to warm up from -33, since the freezer will just stop chilling until the freezer reaches above -15, the time this will take is dependant on how often the door is opened more than anything else.
If the dial is already at the desired position, and the freezer hasnt started warming up please check the action\ fast freeze since switched on, this is indicated by a snow flake on the display. If the snow flake is shown, press the mode button to the right of the display until the snow flake disappears.
Should the snow flake not be shown and the freezer still maintaining -33, it would suggest the thermostat has failed, and require you to arrange an engineer to come to site to investigate further, if you would like me to recommend an engineer, please provide your postcode.