When viewing a soldier in the barracks various options are available:
Units that are injured or fatigued can be given an injection of meld to speed their recovery.
Each injection costs 50 meld and reduces the duration of both injury and fatigue.
The amount of injury and fatigue reduced decreases the more injections a soldier has and is calculated as follows:
% Effectiveness = 80 / (1 + Number of Injections)
You can access meld healing after researching Xenogenetics. Then, after clicking on an injured or fatigued soldier in the barracks, the option for a meld injection should appear.
You can retrain (reset the perks) of any soldier at a rank of LCPL or higher. Doing so will require them to undergo extensive training and they will be fatigued for 10 + 3 days per rank.
Only healthy soldiers are candidates to be retrained.
You can access retraining by clicking on the soldier in the barracks and then accessing their perks. Once there, a button on the bottom right of the screen labelled RETRAIN SOLDIER should appear.
This can be useful if you find a certain build was working for you previously but is not competitive enough in the later game.
Also, if you require a certain type of soldier, you could reset perks to turn, for example, a shooter engineer into a support engineer.
You can retire non-injured idle soldiers from the barracks screen. Retired soldiers will join the Officer Training School and help train your active soldiers.
For every 10 missions (modified by campaign length) that retired soldiers have accumulated in the Officer Training school, every active soldier in the barracks gains +1xp/day.
The amount of missions a retired soldier has stack with other retired soldiers, ergo if you retire a soldier who has completed 7 missions, and a soldier who has completed 3 missions, their total missions would be added together in the OTS to give you a total of 10 missions for xp gaining purposes.
You can view your retired soldiers and the daily bonus xp they are granting by visiting the Officer Training School.