You can repair units by selecting a construction or resurrect unit and use Right Mouse click on any (damaged or unfinished) unit.

You can also use Right Mouse Drag to draw a circle in which every unit will be repaired.

TIP: If you want to keep repairing units, f.e. always within a circle on your front line, you can toggle REPEAT so repairs will always be done.

This is needed because with REPEAR |OFF, when all units are repaired the repair-command will end. If a next wave of enemies roll in and do damage, with REPEAT ON the repairs will recommence again when new damage is found.