There is a number of ways how to do it, check out this thread for a lot of suggested options: Change the '- Any -' text in an exposed taxonomy filter.
You can change the themed output for that form and filter, you need a bit of custom code for that.
You could use Better Exposed Filters module, it lets you override each option text:
Use this field to rewrite the filter options displayed. Use the format of current_text|replacement_text, one replacement per line. For example:
Without any additional modules or code changes you can use the Flagged Exposed filter Grouped filters option instead of the
Single filter you're using now.
The Grouped filters allow you to enter a Label for each grouping. Only in your case you simply select "True" for the first grouping and "False" for the second grouping, which means you haven't grouped anything, you're just repeating the same options from the Single filter: