How do I change the iTunes account user on same device used by 2 family members?

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I Had an iTunes voucher for a gift. Loaded it to my own iTunes account. My husband has ipad2 which doesn't have flash player. So I found flash player for his iPad and followed the link from Apple that takes me to iTunes store page. All good so far. However when I click buy, his account details come up asking for his iTunes pasword (no funds on his account) but with no way of changing login to my login so I can buy him the flash player and download it to his iPad.
I'm trying to do this from his iPad.

My question is how do I change the login details from his to mine through using his iPad and not my laptop where my login comes up automatically as does his on his device?
Anyone plse?