Now you're trying to teach her manners and sitting down during the meal is part of that. That's good. It can be hard, especially if this is a new concept for her. If she's been allowed to be up and running around then she doesn't want to sit. If this has been a long struggle, it's time to bump it up.
You strap her in where she can't undo them.
You swat her hiney and tell her it's meal time and she is not allowed to leave the table until she is finished eating.
You let her get up and then toss her food in the trash and make her feel hungry, really hungry and remind her she got up from the table instead of eating. To me food is not something to battle over but I NEVER EVER allow kids to get up from the table with food. They NEVER EVER eat when an adult is not in the room with them.
I have a lot of years in child care and I have tossed more than one child over my arms to get food unstuck from their throats so I'm waaaaaaay more of a capitol "B" about kids sitting at the table with an adult when they have any sort of food.
When a child is choking is is quiet, silent, nothing. You just notice it got quiet and look around and see them turning blue. So heck no, kids DO NOT get up from my table if they have food.
If she is not chewing or taking food with her and you don't care that she's up then let her go. BUT if you really want to teach her to sit and eat on command then you have to be almost mean about it.
Love and Logic says that if a child never feels what hunger is they don't learn that food makes hunger go away. Food is just something they eat. If they feel hungry and realize food makes that go away then they learn to appreciate food.
Now, that all said.
Battling over food.
My granddaughter is one of the pickiest kids in the world when it comes to food. She will NOT eat when she gets hungry enough. She does NOT eat what I put in front of her on command. If she is forced into taking bites of food she doesn't like she gags and starts puking. She misses days and days in kindergarten because the lunch room supervisor decided she had to be taught a lesson. She tell her to eat her food and she'd say "No, Thank you". Then the lady would bully her into taking a bite by not letting her leave the table with the rest of the kids and make her take a bite. She'd gag and start puking. So they'd send her to the office and make me take off work to come get her. They said she wouldn't eat because she was sick.
I asked her, in front of the principle, if she felt bad and she said no, she wanted to go to class. I'd ask her why she puked and she'd say, every time, the lady made me eat food I didn't like. I finally got the whole story and told the school that they needed to allow kids to say they were done and that lady needed to stop bothering her. Guess what, once that lady went back in the kitchen my granddaughter never threw up again at school.
To this day she will go hungry for the whole day if there isn't anything she likes on the menu.
If you are not fixing foods your child likes to eat then she is not eating because she doesn't want to eat what you're serving. If she isn't getting to choose at least something that she does like and getting encouraged to try new things she will be going hungry and not wanting to sit and eat.
With kids age is a factor too. If she's 14 months she's not going to sit more than a moment or two. If she's 4 and is just being stubborn then she gets swatted and sat down promptly on her hiney each and every time she gets up. Age makes a huge difference. If the battle is over the item being served then there are compromises you can do where meal time is a happier place.