This I the Proper way to convert Stereo Audio Tracks to Mono:
(1) Select Desired Audio Tracks in Cubase Project Timeline
(2) Right Click on one of them → Audio → Find Selected In Pool
(3) In Pool, the corresponding tracks will be selected. Right Click one → Convert Files…
This will open a small window with some options.
(1) Sample Rate
(2) Sample Width
(3) Channels
(4) File Format
(5) Options that allow you to replace the files in your project or make new tracks
See this JPEG
EASY PEEZY!!!