Click on the "Upload" button and choose the file you wish to upload (in .txt or .csv).
Do note that the number of keywords you can upload at a single time is limited by your plan:
After you have added a bunch of keywords to Keywords Explorer (or uploaded a list of keywords), you'll see the Overview section with the keywords you entered and their advanced metrics/stats (see image below).
Here you'll have the option to export this list.
Do note that 1 credit is taken for fetching advanced metrics for each keyword, from your "monthly SERP updates limit":