Split Unit button

a new window will pop up

there you can decide how much strength will be given to the new unit.
after that you can choose where the new unit will be deployed

To merge two units :
they must not be moved, must be understrength, must be same type and level
Choose a unit and select Merge units icon.

after that all units that is possible to merge with will be highlighted

After selecting the target unit it will be reinforced to full strength.