Hello Julia,
To set Office 365 for the web as the default app for opening Word, Excel, and other files, you can try the following steps:
Click the Start menu and search for "Default apps".
Click "Default apps" from the search results.
Scroll down and click "Set defaults by app".
Find "Office Online" in the list of apps and click on it.
Click "Choose defaults for this app".
Check the boxes for the file types you want to open in Office 365 for the web, such as .docx for Word documents or .xlsx for Excel spreadsheets.
Click "Save" to apply the changes.
After completing these steps, any files with the selected file types should automatically open in Office 365 for the web when you double-click them.