I used bitcoinj library for wallet and it generates 12 word mnemonic seed.
But I made changes in it and created 24 word mnemonic seed.
But I need to convert old 12 word mnemonic seed into new 24 word mnemonic seed.
Please help me with this.
This is not possible.
The mnemonic words are an encoding of the entropy used in the seed, essentially. A 12 word seed has less entropy than a 24 word one, so you cannot get 24 words out of it. Similarly, you cannot get a 24 word seed to turn into a 12 word one.
Your best bet is to generate a new 24 word seed and move funds over.