If you have Excel installed on your computer, you should be able to open .xlsx files directly. If you do not have Excel installed, you will still be able to open the .xlsx file in Google Sheets. To import data:
Open the file menu and select the "Import" option.
Google Sheets file menu
Select the "Upload" tab in the Import file window. Drag and drop your .xlsx file into the window or navigate to it using the file browser.
Import interface. You can drag-and-drop files in, or select them with a file browser.
Once you've selected a .xlsx file to import, Sheets will ask if you want to insert or replace (recommended) your current spreadsheet. Either option will work, but "replace" will overwrite the blank spreadsheet created upon generating a new Google Sheets file. Unfortunately the plots will not appear as intended, but you will still have access to the raw absorbance vs. wavelength data.
Import file options
Example data displayed
If, for some reason, you cannot import the .xlsx file, you can try importing the .csv file. If you open a .csv file, it should open as a spreadsheet (plots are not included). If you want to open the file in Google Sheets, you will have to import it. In the Import file menu there will be an option for separator type, under which you should select "comma." Then click "Import data" and you should be able to view the .csv as a spreadsheet in Google Sheets.
File import menu with options for separator type