Once you have bought your smallcase, and want to increase your exposure to it - you can invest more in your smallcase.
From your Investments page, click on any smallcase > Invest More on the right > Confirm Amount > Invest More > Confirm Orders.
The Invest More option helps you buy more of what you currently hold, that is, it will have a minimum amount to maintain the weighting scheme. You can increase this amount (not necessarily in multiples) to invest whatever amount you like, and it will be adjusted to the closest amount required to buy the stocks in whole number of shares based on the weighting scheme.
You can use this functionality to double down on your investments anytime you like. You can also invest more regularly through SIP.
You can also place this order for your smallcase after the market hours using AMO. The order will be executed in the next market day between 09:00 AM and 09:14 AM (pre-market hours)
For more on how to place AMO orders, click here
Note: Please maintain a surplus cash of some additional amount in your Zerodha funds. This is to ensure you have enough funds to purchase all the stocks instantly in case if the stocks price hiked up.