Ok, you can solve it.
Create a backup of admin/match.php (so if you have problems you can start again)
Open this file with notepad
Search this code (row 182):
<?php if (( $cup && !$is_finals) || ($mode==”add” && !$is_finals) ) : ?>
and remove it
Search this code (row 167):
<th scope=”col”><?php _e( ‘Day’, ‘leaguemanager’ ) ?></th>
<?php endif; ?>
and below add this code:
<th scope=”col”><?php _e( ‘Match Day’, ‘leaguemanager’ ) ?></th>
With Larsebald suggestions (row 182) and my changes (row 167) you should be able to change and save the match_day when editing matches.