You target them and use the power once. You can see what upgrades (and other (de)buffs) the henchmen have by clicking the small arrow at the right side of your pet window.
You need to pick enchant undead, this will upgrade your zombies. it doesnt stack so only do it once for each. the recharge is a bit naff, so best some recharge reds on there. you will get to choose your 2nd tier of pet at lvl 12 i believe, which is grave night, which is very cool. you can tell you upgraded him, beacuse when you do, armour will appear on him. you will get your 3rd zombie at lvl18 i think. and later you will get your lich, which rocks quite frankly. hope this has helped.
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You need to pick enchant undead, this will upgrade your zombies. it doesnt stack so only do it once for each. the recharge is a bit naff, so best some recharge reds on there.
[/ QUOTE ]Not really, the first upgrade recharges fairly quickly, it's fine with just one rech/end in the default slot.
The Upgrade power is available at level 6, and as stated before, just target a henchman and use it once on each 
You get another upgrade power at lvl 32.
I highly recommend NOT adding any slots to the level 6 power, it recharges pretty fast for a power you only have to use 2 or 3 times a mission (unless your pets keep dying) , so just stick 1 Recharge enhancement in there and you are good to go
[The lvl 32 power takes aaages to recharge, so if you get spare slots later on, you can put another couple in there to speed it up ]
MY personal choice was to pick Hasten. It reduces the recharge time for all powers and is particularly useful for recharging Dark Empowerment (the lvl 32 "upgrade").
So, at the start of each mission I switch on Hasten, summon all my henchmen (3 zombies, 2 Knights and the Lich), cast the first upgrade on each pet, the 2nd upgrade on the knights and Lich then I have to Rest before then summoning Dark Servant and then proceeding.
You get into a routine with all this so it doesnt really take too long. I then cast Dark Empowerment on the zombies as, and when, it recharges.
With Dark Servant and all my pets fully upgraded, they all have good attacks, all can heal themselves and are so lovable and cuddly.. ok, forget about the last bit.
Hey Just started mm tonight with zombies. I have 2 now and was just wondering how exactly do I "upgrade" them? :P
** don't kill me I'm a newbie ** or do they upgrade automaticly?
I got a power to do with upgrading not sure if it worked?