Export the Products (of Product Type Goods and Track Inventory By Quantity) relevant to you from the list view, i.e. by navigating to Sales -> Products --> Actions -> Export using the option I want to update data (import-compatible export): and ensure the field Quantity on Hand is added to the list of Fields to export:

Then, simply set the quantities you want to import in the column qty_available in the file just downloaded (and save it!):

Finally, import the file again via Actions -> Import records, hit Upload Data File and select the file you've just updated with the Quantities on Hand and lastly, Test (optional but highly suggested) and Import:

Please note: This way of importing Quantity on Hand is adding up Quantities already present! I.e. if you happen to import the same file (or product) twice, you will end up with double the Quantity on Hand!

General information about the import can be found here https://www.odoo.com/documentation/18.0/applications/essentials/export_import_data.html  and here https://www.odoo.com/documentation/18.0/applications/sales/sales/products_prices/products/import.html 

Note: This method assumes each product has the same cost. If you have products with different costs, you can instead import a Receipt (Warehouse Transfer) which gives you the ability to specify the Unit Price of the quantities you are  importing.