Its more easy then people think. Seed is so delicious to them they just eat that till they are full and so don't even want to bother trying new food! If we were full after eating pizza we would not really want to try some strange exotic food we had never seen before.
Cut down on the seed. Once its night time and they are sleeping you should take the seed out of the cage.... in the morning they will be hungry .. and that is the best time to add the new foods you want them to try..............
I found my birds LOVED hard boiled egg.. smashed up (with the shell). I used to finely cut up broccoli, cabbage, grated carrots, etc and mix it in with the egg. (you can even put in a few seeds at first to get them interested). You can feed them some of this every day and keep the rest in the fridge. ( Don't give them more than they can eat.. and don't leave fresh foods in the cage for hours.. or they may go bad).
they will start to sample it as they are so hungry.. once they get used to it after a few days they will love it and even prefer it to the seed!! Once they have sampled the new food.. even if they only pick it up or play with it at first,.. you can then put their seed back in the cage.
MORE TIPS....
Just give them a small spoonful of seed in the morning. Don't give them any more seed until they have eaten nearly all of it.. even the seeds they don't like. (they will always leave a small amount of stale or gone off seed).
The seed should be mostly gone by the afternoon.. then you can put some grated and finely chopped up greens.. in their feeder.. sprinkled with a SMALL amount of seed. they will start eating the seed and sample the fresh vegetables too.
Most people waste so much seed by constantly topping it up and the birds just rooting through it picking out their preferred seed.. throwing the rest on the floor. Over time the birds will get sick.. as they are only eating one of two types of seed.. and this will be lacking in nutrients. Over time you will know roughly how much seed they will eat in a day.. Never top up a seed feeder until its nearly empty.
Good luck.