Hi Julia, I was lucky in the way that my boy Mytsique learned to eat most fruits and veggies quite quickly.
When Mystique was 8months, I got another female budgie who I named Fantasy, and they bonded quickly and she learned to eat things other than seeds from him.
But before all that, I got Mystique accustomed to eating other food when I had him out. He is handraised and trusts me, so I first offered him a bean, and he didn't eat it, soI took a bite, then offered it back to him. This time he approached it and looked more interested, but still wouldn't bite it. So I took another bite (be careful though as human saliva is detrimental to budgies, I ate it from the other end). Eventually he began eating beans and absolutely loves them now.
Another favourite of my birds is broccoli.
They eat it like they do seeds haha, they roll the little pieces in their beaks.
With this, instead of eating it myself, I pinched my pointer finger and thumb together to kind of resemble the beak of another bird. I then broke off pieces of the broccoli using the "beak" and I rolled them in between my fingers. They watched curiously and it seemed to work straight away for them. It really helps if they trust you.
Hopes this helps and good luck !