My TH is now level 6, and the laboratory is level 4. About upgrading troops and spells, please tell me the priority order.

Giant, wizards and archers

In town hall 6, the laboratory can be upgraded to level 4.
In laboratory 4, troops that can be upgraded are the giants, wall breakers, balloons and wizards.
Spells that can be upgraded on the other hand on level 4 laboratory are the heal spell and the rage spell.
All these upgrades are from level 2 to level 3 for both troops and spell.
About the priority order, it best to upgrade your giants first among these selection of troops.
It is important to upgrade troops that you most commonly bring to war.
Upgrading giants will cost 250,000 elixirs, one of the low costing troop to upgrade in this level together with the wall breakers.
After upgrading the giants, you may want to upgrade the wizards next.
Then upgrade your heal spell and rage spell accordingly.
The spells require more elixirs to upgrade compared to that of your troops.
The heal spell costs 600,000 elixirs while the rage spell is at 900,000 elixirs.
The next on the list will be the wall breakers, and then the balloon.
All troop upgrades will take two days to finish including the heal spell.
It is only the rage spell that takes three days to upgrade.
Make sure you have enough resources before each upgrade finishes to follow it with another upgrade.
It is best that your laboratory can research one after another right away.