How do I sell shares received in IPO allotment in minor demat account?

There are two ways:

1. Sell them directly through the minor trading account linked to the minor demat account in which you received the allotment. The process is same as any other trading account.
2. Transfer the shares to the Guardian's trading account and sell them.

Note that as tax liability lies with the account Guardian, it actually doesn't matter where you sell it from.