Could not activate my car alarm

Question edited for clarity.

It senses changes in air pressure.

Interior volumetric. The system checks for any variation in
the volume in the passenger compartment. The alarm is triggered if anyone breaks a window, enters the passenger compartment or moves inside the vehicle. Self-protection function The system checks for the putting out of service of its components.
The alarm is triggered if the battery, the central control or the wires of the siren are put out of service or damaged.

You can deactivate it.

Deactivation of the interior volumetric protect
Switch off the ignition.
Within ten seconds, press button A until the indicator lamp is on continuously.
Get out of the vehicle.
Lock or deadlock the vehicle using the locking button on the remote control.
The exterior protection alarm alone remains activated; the indicator lamp in button A flashes once per second.
To be effective, this deactivation must be carried out each time the ignition is switched off.

https://www.peuclub.com/pgcont-1299.html

The manual shows 'Button A'.

https://www.manualslib.com/manual/670948/Peugeot-308.html?page=89

..
..

Pull the mains plug for 10 minutes. Don't open the doors, plug it back in. That should reset it.

There are different alarm systems, not same for all kind of cars. You should explain, by the way, what do you mean by "resetting" an alarm.
If you mean that you have an activated alarm and you want to deactivate it some can be deactivated by turning the start key.
Then, some have a hidden button you may press
Finally, some CAN'T be deactivated, only by remote.
You only can turn off the sound removing battery cable then deactivate physically the system and to do this you should know alarm installation techniques.
To change alarm coding you need and electronic tecnician.

1 Turn the ignition key counter clockwise in the driver's side door to unlock the car. This deactivates the alarm long enough for you to gain access to the vehicle. Then, place the key in the ignition and turn the key until it clicks once. This starts the car's battery, but doesn't ignite the engine. The LED light on the dashboard should stop blinking, indicating the alarm is temporarily disabled.

2 Locate the box for the alarm under the dashboard on the driver's side of the car. Then, look for the override button on the box. Typically, the small button is either red or black. With the ignition key in the "on" position, press the override button and hold it in until the alarm chirps once. This disables the alarm until you manually reactivate it.

3 To reactivate the alarm, turn the key to the "on" position and press the override button once. The alarm chirps twice, indicating the alarm is active.

Hope this helps!

If you have a factory installed alarm the original factory key can activate the alarm by pressing the lock button twice(rapidly). The lock symbol is generally a symbol of a "locked" lock.
If there is no buttons on the car key or it is an aftermarket alarm, a key fab (a little alarm remote control with 2-4 buttons the size of a keychain) is needed to activate/deactivate the alarm.

pressing lock button should activate alarm sound. system, when you press unlock button that deactivates alarm sound.i you dont hear alarm sound problem is in the ecm.

Sorry you can't disconnect it. you should be able to reprogram it though. look in your owners manual. most are programmed threw the radio controls.

System operation
Pressing remote control lock button locks vehicle and activates system Fig. 1 . Indicators flash once. Pressing remote control unlock button unlocks vehicle and deactivates system Fig. 1 . Indicators flash twice.
Fig. 1
Synchronization
When
System malfunction. Remote control added or replaced.
How
Access alarm system control module located under driver's side lower fascia Fig. 2 . Set memory switch to SET position. Press remote control lock or unlock button. Set memory switch to OFF position. Confirm synchronization has been successful by locking and unlocking vehicle. Repeat procedure for second remote control. Synchronization can also be carried out using diagnostic equipment.
NOTE: A maximum of 2 remote controls can be programmed. NOTE: If remote control fails, system can be deactivated as detailed in the owner's handbook.

depending on model version and age.
the left stalk switch on the steering column is for the turn indicators
the emergency flasher lights or hazard lights is either between the
air vents on the dash above the middle console or at the bottom of the middle console below the radio and above the ashtray is is oblong button
both have a red triangle on them. press once to activate and press once again to deactivate.

SYSTEM OPERATION

System warning lamp located on top of the instrument panel
Press UNLOCK button once , turns on interior lamps and deactivates alarm (if fitted)
Press UNLOCK button twice w/in 5 seconds unlocks all doors, turns on interior lamp and deactivates alarm (if fitted)
To activate alarm system (if fitted):
Switch ignition OFF
Open driver's door
Press Lock button on remote
Close Driver door
warning lamp will flash when system is activated
Press remote power sliding door button will open sliding door (if fitted) only if unlocked first
Pressing remote panic button:
*Sounds horn intermittently
*Flashes exterior lamps
*Turns on interior lamps
**Pressing panic for the second time OR switching ignition ON deactivates alarm.


PROGRAMMING
Remobe key from ignition switch
Remove the "BCM PRGRM" fuse, from the fuse box
Close all doors
Turn ignition switch to the ACC position
With in 1 SECOND , turn the ignition switch to OFF then back to 'ACC'
(really, with in one second)
Open and close the driver's door. A chime will sound, indicating programming mode activated
Press and HOLD LOCK and UNLOCK SIMULTANEOUSLY untill a chime sounds 2 times ( about 15 seconds)
Repeat this exact procedure for all remotes to be programmed
Remove key from ignition
Refit 'BCM PRGRM' fuse.

4 REMOTES MAXIMUM CAN BE PROGRAMMED