An M1 user, always has complete control of their portfolio and can make edits to your slices (stocks, ETFs, Pies) at any time. When editing your portfolio, you have the power add, remove or alter the target percentages on slices.
In order to edit your portfolio, you will have to first get to your Pie Details page, to get their click the 'view details' button on the portfolio page.
To add a slice, click the 'add slices' button. You will then choose any new stock, ETF, or Pie to add to your portfolio.
After you choose your new slices, they will queue up at the bottom of the screen. When you are ready to add them to your portfolio, go ahead and click add to pie.
To remove a slice, on the 'Pie Details' page, click the box next to a slice and then click remove. Removing a slice will trigger M1 to sell that slice during the next trading window.
To update your target percentages, change the percentages to the new targets and click 'save changes'.