Yes, you can.
There are two ways to edit a size chart:
1. Fast and Easy
Dashboard: click on chart > Editor: make the needed changes
We recommend this option if the change is small and you have low traffic in you website. This is because, any change you do on the Editor will be automatically be visible to your client, even if they are not final.
2. Slow
Dashboard: click on chart menu: click on duplicate > Editor: make the needed changes > Link to Products: select again the products you wish to link > Dashboard: click on publish
This option is recommended if you need to make significant changes to the size chart and don't mind re-doing the linking to products and collections.