smallcases are flexible instruments that can be managed anytime. You have the provision to buy & sell individual stocks or some shares using the Manage option.
To sell a stock completely, reduce the desired quantity to 0 and all the shares will be sold off.
To add a new stock, use the search box option to find and add a new stock.
Adding a stock or removing using the Manage option will deviate the smallcase from the original theme. You will still receive the rebalance updates for your smallcase even after managing. But the changes made will not be included in the rebalance updates.
To place/complete a manage order, you will need to have the complete funds required for any included buy transactions in your usable margins. This is in effect from Aug 1, 2020. You can read more on it here.