I'm already in the USA since 2 weeks. Can I extend my stay by a month when I told the CBP I'd only be there for a month

Short answer: you cannot extend your stay.

Long answer: as @Traveller points out above, the "WT" on your passport denotes a person who has entered the US temporarily for travel purposes under the Visa Waiver Program. The USCIS webepage cited in that Comment is titled "Extend Your Stay," and contains the following text:

Extend Your Stay

...

If you want to extend your stay in the United States, you must file a request with U.S. Citizenship and Immigration Services (USCIS) on the Form I-539, Application to Extend/Change Nonimmigrant Status before your authorized stay expires. If you remain in the United States longer than authorized, you may be barred from returning and/or you may be removed (deported) from the United States. Check the date in the lower right-hand corner of your Form I-94, Arrival-Departure Record, to determine the date your authorized stay expires. We recommend that you apply to extend your stay at least 45 days before your authorized stay expires.

You may apply to extend your stay if:

• You were lawfully admitted into the United States with a nonimmigrant visa
• Your nonimmigrant visa status remains valid
• You have not committed any crimes that make you ineligible for a visa
• You have not violated the conditions of your admission
• Your passport is valid and will remain valid for the duration of your stay

You may not apply to extend your stay if you were admitted to the United States in the following categories:

• Visa Waiver Program [emphasis added]
• Crew member (D nonimmigrant visa)
• In transit through the United States (C nonimmigrant visa)
• In transit through the United States without a visa (TWOV)
• Fiancé of a U.S. citizen or dependent of a fiancé (K nonimmigrant visa)
• Informant (and accompanying family) on terrorism or organized crime (S nonimmigrant visa)

Because you entered the US on the Visa Waiver Program, you are not eligible to extend your stay. You should depart the US no later than the "admit until" date. Not complyinig with the "admit until" date will make it immeasurably more difficult or impossible for you to enter the US in the future.

You can almost certainly extend your trip to the US, yes!

When you entered the country you would have been given a date that you must depart the country by. This would normally be written into your passport over the entry stamp, but can also be accessed via the CBP I94 website (select "Get most recent I94").

This will show an "Admit Until Date" which is the date that you must leave the country by.

With very few exceptions, when entering the US under the Visa Waiver Program (VWP) as you did, you will be given 90 days of admission to the US - even if you stated at time of entry that you were only staying a money.

You can change your return flight to be any time up until the day listed as the Admit Until Date without any consequences, and without any need to inform the US Government. As someone that entered under the VWP you can NOT extend your stay any longer than this date.