This is easy. Just click the Paid Accounts link on the menu side-bar. Choose the level of Paid Account you want to buy for someone, enter that user's journal name, and click Buy Now. If you want a specific gift message to be sent to the person's address on file, put "Gift Message:" in the notes/additional instructions area in the checkout forms and include your gift message.

If you don't include a gift message, the journal user will be informed of their upgrade, but the gift notification will be sent with the giver as anonymous.