What is the solution when i want to update list data from Application to exists large records in SQL server (more then 800million records)

Please tell me how to update list data from Application to exists large records in SQL server (more then 800million records).

This mean : i will foreach my List data --> then [select record by id from table in SQL] then if exist record i will update record from Application to SQL server. Because the database very large then 1 record want to be updated will find more than 800million records in SQL--> 1*800million--> so i want to update 100 record then --> 100*800million.

Please help me the best solution to solve this prolem. Thank you so much!