You can do it manually.
Go to Settings - Player - Subtitles. Enter Position on screen. Choose Manual.
Start Video. Enter Settings - Video Settings. Choose Video calibration. Set the short bar down to your choosing. Save calibration. Hit back. Enter Again Video Settings. Choose Set as default for all media.
Or if Video calibration doesn’t work, @Sholander has a solution described here CoreELEC 20.0-Nexus_rc2 Discussion - #66 by Sholander