Updating nodes from multiple xml strings

i basically need to encrypt the data that is stored in field of a table as xml string. I am using AES encyption. I already have the logic for encryption and decryption . All I want to do is load the xmls string and update two nodes. Basically I need to update 23232323232323 nodes. I basically have around 60,000 records and want to do it efficiently using c# program. Please see the sample of the xml string below

 <?xml version="1.0" encoding="utf-16"?><CallValidateResponse xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema"><Result><RID>da4eee59-7abc-4ff0-b057-41ff823549f8</RID><PID></PID><Displays><BankCheckEnhanced><Result></Result></BankCheckEnhanced><BankCheckStandard><Bacs_status></Bacs_status><Transactions_disallowed_DR></Transactions_disallowed_DR><Transactions_disallowed_CR></Transactions_disallowed_CR><Transactions_disallowed_AU></Transactions_disallowed_AU><Supervisory_body></Supervisory_body><Major_Location_Name></Major_Location_Name><Minor_Location_Name></Minor_Location_Name><Branch_name_or_place></Branch_name_or_place><Full_branch_title_part_1></Full_branch_title_part_1><Address_line_1></Address_line_1><Address_town></Address_town><Post_code_major_part></Post_code_major_part><Post_code_minor_part></Post_code_minor_part><Full_name_of_owning_bank_line_2></Full_name_of_owning_bank_line_2><Full_name_of_owning_bank_line_1></Full_name_of_owning_bank_line_1><Short_name_of_owning_bank></Short_name_of_owning_bank><Short_Branch_title></Short_Branch_title><sortcode>0</sortcode></BankCheckStandard><IdentityCheck><Appverified>false</Appverified><Sdnwarning>false</Sdnwarning><Sanctionswarning>false</Sanctionswarning><Deceasedwarning>false</Deceasedwarning><Goneawaywarning>false</Goneawaywarning><Pepwarning>false</Pepwarning></IdentityCheck></Displays></Result><Errors /></CallValidateResponse>