You haven't stated why the builder is withholding handing over possession. Is the Occupancy Certificate issued by the competent authority in place? Changing locks is not advisable. Better issue a legal notice demanding possession immediately, and take further steps according to the outcome, in consultation with your lawyer.

You cannot change the locks 

builder can deliver possession only on receipt of occupancy certificate 

ask builder whether OC has been issued or not 

First of all getting the property registered without obtaining occupancy certificate itself is not advisable, you have not stated that if the OC has been obtained or not.

Registration of property alone is not sufficient, the builder has to hand over possession with a possession letter duly signed with his seal hence until then you cannot change the lock, iot will be considered as an offence.

You can issue a legal notice demanding possession of the property.

The reason of handing over the possession is not known. If you are the lawful owner then you can break open the lock and take possession of the property. 

- Occupancy Certificate evidences the completion of the building as per the approved plan and compliance of local laws.

- If the builder is unable to obtain the occupancy certificate, it means that the builder has violated building law or deviated from Construction Plan, and further it is illegal to occupy in a building or apartment that doesn’t have an occupancy certificate.

- Further, It is illegal to occupy in a building or apartment that doesn’t have an occupancy certificate. Buildings without OCs are not given a water connection and are charged a higher property tax.

- Hence, in the absence of an Occupancy Letter, you should not accept the possession letter from the builder.

- Further, you should not pay the full amount before handing over the OC to you. 

- Further, as per RERA,, A builder cannot give possession to buyers without OC

- Hence, if the builder fails to give or obtain OC, then you should lodge your complaint before RERA.

- Breaking the lock is not suggested without a property complaint against the builder. 

You can send a legal notice to him. Directly changing locks without possession is not a good idea

1. Occupation of builder is illegal and an offence unlawful restrain.
2. File a criminal complaint in the Court of Magistrate  against him.
3. Approach District Consumer Commission under new Consumer Protection Act, 2019, under which proceedings are very fast.
4. You can claim reimbursement of rent, compensation for mental agony, costs of complaint and lawyer's fee and you will all.

Yes, since you have the sale deed and mutation in your name, you are the legal owner of the property. If all dues are paid, the builder cannot withhold possession arbitrarily.

Steps You Can Take:

1. 
   Issue a Legal Notice – Send a written legal notice demanding possession and a No Dues Certificate.
2. 
   Change Locks If Needed – If the property is legally yours and no possession is pending as per agreement, you can change the locks but ensure no illegal trespassing or forceful entry.
3. 
   File a Complaint – If possession is denied, file a complaint with RERA (if applicable) or in civil court for possession and damages.
4. 
   Police Complaint – If the builder tries to obstruct possession forcefully, file a police complaint for criminal intimidation and breach of contract.

For detailed, personalized advice, consider a phone consultancy. Hope you find the information helpful. You are free to contact me for further discussion. If you could spare two minutes of your time to write a review, it would be greatly appreciated and bring immense happiness to read it. Thank you. Shubham Goyal.

Sir/Madam,

You are suggested not only to change the locks but also file the police complaint against the builder.