If a player is substituted out of a match because they suffer a concussion, go to Scoring -> Match Details/Teams, highlight that player on their team list, click the dropdown arrow on the Replace button and select Concussion Substitute.
This opens the normal player list, with the other players already in the Playing XI greyed-out to prevent their accidental selection, so select the new player and click Add (or double-click the player) to add them to the team list. The substituted player will then be greyed-out and the substitute will be available in all dropdown lists.
If a player is substituted because they have been legitimately replaced in a match, e.g. by being called-up into a higher-level match, repeat this process but select Substitute instead. If a player was wrongly included during match setup, use the Replace function as normal.
If these substitutions have been done incorrectly, or not at the time/over they occurred, or are the wrong type, click on the Player/Role Substitutions icon in the bottom-left corner of the Edit Match window to open this dialog:
Double-click on the required cell to reveal its dropdown arrow, then use this list to make the correction, or highlight the row of an incorrect substitution and click Delete.