If you are taking out gravel, it's a good idea to take out the fish first. Changing gravel really pigs out the water. You can use a net to take it out, or a clean plastic cup with some holes punched in the bottom.
I don't have fish in there yet, but I am going to put a turtle in there for cycling. Turtles might eat gravel, should've though of that earlier . I don't want to change all the water again, how would I go about doing this?
mmm... if there are no fish in there, nor a turtle, then it would be easier to change all the water or like 50% , you don't need to cycle the tank for your turtle so new water wont be a problem, it will be easier with less or no water..... or do what Tolak said, seems to be a good method