You have two ways of allocating more tokens to your channels, either:
This option will let you distribute tokens across all open channels for a token network.
Click the "ADD FUNDS" button in "Tokens" screen next to the token network you want to allocate more tokens for.
This option will let you allocate more funds to a specific channel. You can compare this process to "topping up" a prepaid card.
Click the "DEPOSIT" button in the "Channels" screen next to the channel you want to deposit more tokens in.