try this:

Manual Defrost Mode

When pressing both UP and DOWN keys for 3 seconds while on setting "2",

the control will enter Manual defrost mode.

The display will show a "d".

Manual defrost can be cancelled by again holding both UP and DOWN keys for 3 seconds.

Manual defrost mode will terminate after the defrost cycle has completed.

The control will resume normal operation on setting "2".

other Frigidaire Freezers:

To manually defrost the freezer, press and hold the extreme freeze and the temperature alarm buttons simultaneously for 10 seconds or more.

A half second beep will sound and “df” will appear in the display for 3 seconds

indicating the control has entered into the 30 minute defrost cycle.

At the end of the defrost cycle, the control will then restart its 12 hour compressor run time count

for the next automatic defrost cycle.

Manual Defrost Mode

In the event that a manual defrost is desired, begin by pressing the AIR FILTER and ALERT RESET keys for five

seconds.

The display will show “dE” and the audible confirmation tone will be heard.

The “dE” will be displayed for the duration of the manual defrost cycle.

The defrost cycle terminates automatically after thirty minutes and then the display will show the set temperature.

The HIGH TEMP alarm will be inhibited for 1 hour beginning at the start of the cycle.

Manual defrost may be cancelled at any time by pressing the AIR FILTER and ALERT RESET keys for five seconds.

To learn more about your freezer or to order parts, click here.

Source: HOW DO I INITIATE A DEFROST CYCLE ON A FRIGIDAIRE FREEZER LFFU14F5HWH