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 (for plans started after March 2022 the limit is 500 updates with each Subscription plan"):