Go to File > Save As.
Choose CSV (Comma delimited) (*.csv)
the option like Import or Import Records (this is usually found in the top right corner of the leads list view)
Odoo will prompt you to map the columns in your CSV file to the corresponding fields in Odoo.
If there are any errors, Odoo will usually provide feedback on what went wrong. You may need to correct your CSV file and re-import