If you have an existing spreadsheet you created in another program, you can easily import and convert it to Sheets.
Go to Drive.
Click New > File Upload. Then, choose the old spreadsheet from your computer. Supported files include .xls, .xlsx, .xlt, .ods, .csv, .tsv, .txt, and .tab.
Right-click the spreadsheet you want to convert.
Select Open with and choose Google Sheets.
Converting your spreadsheet from another program creates a copy of your original file in Sheets format. You can then edit it in your browser like any other Sheet.
Have an Excel spreadsheet?
If you've already stored Microsoft® Excel® spreadsheets in Drive, you can also update them without converting to Sheets: