Channel:Form Updating

I am using the grid field as a custom field for one of my channels. My form is setup correctly from what I can tell. What happens is, when trying to add to an entry and already having a few grid fields worth of data in that entry. After I submit the new entry and wanting the grid field to just update and leave the other grid data alone in that channel entry, but it replaces all of my data with just the one newly submitted record. How do I go about simply modify and adding onto data already present in the Entry with my Grid field?

<input type="text" name="field_id_10[rows][new_row_0][col_id_21]">