If "kneel" means to bend the knee in a position of respect or submission:
* Stand
* Rise
* Erect
* Upright
If "kneel" means to rest on one's knees:
* Sit
* Stand
* Lie down
If "kneel" means to fall to one's knees in submission:
* Resist
* Defy
* Challenge
* Stand up to
The best antonym will depend on the specific situation and the intended meaning.