You can trim a plant and replant the top part again - this is referred to as "topping". With most stem plants, like your L. indica, they can be trimmed back. You can remove the entire plant from the tank, cut it off near the bottom to shorten it a bit, and replant in the tank. You'll be cutting the roots away but that's ok. It will quickly grow new roots. You may have to trim like this once a week if it's a fast grower. Like Travis says, you can also leave the bottom portion in the substrate and reach into the water to cut the stem a few inches (or however much you want) above the substrate. You'll have the rooted part still in the substrate - this part will still grow. You can also replant the cut part (the top) and it will grow too. Most people like to replant the top part since it is the prettiest part of the stem. I will get too many plants if I keep the bottom, rooted part, so I top and replant, and throw the root part away.