JSON.serialize() method can be used to generate JSON String from Salesforce Records Update.

Sample Trigger to generate the JSON Payload:

trigger Accounttrigger on Account ( after update ) {
    
    List < Map < String, String > > payload = 
        new List < Map < String, String > >();
    
    for ( Account objAcc : trigger.new ) {
        
        Map < String, String > mapContent = 
            new Map < String, String >();
        
        Account oldAcc = trigger.oldMap.get(
            objAcc.Id 
        );
        
        if ( 
            objAcc.AccountNumber != oldAcc.AccountNumber 
        ) {
            
            mapContent.put(
                'AccountNumber', objAcc.AccountNumber
            );
            
        }
        
        if ( 
            objAcc.Description != oldAcc.Description 
        ) {
            
            mapContent.put(
                'Description', objAcc.Description
            );
            
        }
        
        if ( 
            mapContent.size() > 0 
        ) {
            
            mapContent.put( 
                'Id', objAcc.Id 
            );
            payload.add(
                mapContent 
            );
            
        }
        
    }
    
    if ( 
        payload.size() > 0 
    ) {
        
        System.debug(
            'payload is ' + 
            JSON.serialize(
                payload
            )
        );
        
    }
    
}

Sample Apex Code to update Account records to test:

List < Account > listAccounts = [
    SELECT Id, AccountNumber, Description
    FROM Account
    LIMIT 2
];

for (
	Account acc : listAccounts
) {
    
    acc.AccountNumber = 'Test';
    acc.Description = 'Sample';
    
}

update listAccounts;

Output: