Hi Chethana,
It is possible when doing an ImportfromCSV for new records and existing records to be updated. The trick of that is the key between the two tables needs to be consistent , there needs to be a unique identifier that they share in order for the API to have the necessary logic to combine records or create a new record. In the ImportfromCSV call you would want to take advantage of the mergeFieldId parameter, you can find more in the help doc for 
API_ImportfromCSV in our API Guide
| mergeFieldId | With this parameter, the import uses the field specified as the key, instead of the key field for the table. 
The merge field must be a unique field, and the specified merge field must contain unique values.
Values for the table key and/or record ID field will be ignored from input to prevent data corruption.
If the specified merge field uses a formula, and a record cannot be found by its value, a new record will be added if the resulting formula value is a valid value for the field.
Record ID or other table key field can be used as the specified merge field.
The clist parameter is required if this parameter is used. To specify the field, use the field ID, for example: <mergeFieldId>10</mergeFieldId>
 For more details, read an article about the parameter on the Quick Base Community site. | 
I hope this suggestion is helpful Chethana.
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Evan Martinez
Community Marketing Manager
Quick Base
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