In Legends of Kingdom Rush we have two kinds of Heroes: Legendary and Companions. Your party will comprise one legendary hero, and three companions (two that you can choose on the party selection screen, the other one will be recruited at random during your adventure).
How to change your Party Members
Legendary heroes are framed in gold on the Party Selection screen and will be the first character at your party. You can choose only one Legendary Hero.
Then we have the Companions, which are the heroes not framed in gold.
This includes ranger, arcane wizard, knight, bombarder, barbarian, dark knight... and so on.
To switch Companions, just select which one you want and press EQUIP.
The game will highlight your current companions, allowing you to choose which one you want to replace.
And all we're all done!