How can I delete my old/weak units ?

At the moment the only way to selectively delete units is while they are wounded in the hospital. Place your unwanted units in an APC formation and let it attack zombies. Once they are wounded and in the hospital go to the Heal screen but instead of healing tap on the icon for the troop type (on the left) and then select the troops to 'Dismiss'.

Attacking other players will also injure and in some cases kill (remove) your troops but you may find you lose ones you want to keep as well.

Just be aware that even though you can dismiss these troops, they will still use up hospital capcity and use resources. So technically .... lol you can't truly get rid of them