Creating windows service for bulk upload through excel in asp.net c#

What DBMS are you using?

I can share a design approach for this problem statement.
1) Create a Table say ExcelImportFiles, in Database to store the Excel file as varbinary object, with a flag as Uploaded.
2) Create a Windows Service with a permissible value for thread sleep to get a looping behavior. In service, write code to read the file from the ExcelImportFiles Table and create a excel file. Then read the Excel Data in a DataTable or any DTO object. And then once you have performed the required validations if any, update the data in your Database either using a SQL Command, EF, or any other ORM.

Basically you should create a queue to process your excel files, this would be an extra over head for you. But if you are using SQL Server Database, then you can use SQL Server Broker Queue for the queue logic and focus on the Excel Import feature.