$ mplayer Lecture_video_part1.mkv
$ ^1^2^
mplayer Lecture_video_part2.mkv

This command will replace 1 with 2 in the previously executed command. It will only replace the first occurrence of the string and is equivalent to !!:s/1/2/.

If you want to replace all occurrences, you have to use !!:gs/1/2/ or !!:as/1/2/.