Okay, so you need to use the update collection Service, and invoke it on the on the button clicked.
So create a new database service and pick update service on the collection name. When the item is returned, you need to capture the _id and put it in localStorage or a label thats not visible.
using the _id, you use the service and in the URL you need to add {id} at the end.
precode
https://api.appery.io/rest/1/db/collections/<collectionName>/{id}
/code/pre
In the request tab of the service, add 'id', and map the _id that you just stored to the value of id. Now it knows what object (row) it needs to update.
Then you need to add another request param 'item', and then map the populated edited text box to it.
Thats it! When the user visits that page, with the 'item' in the text box, it stores the _id. When the user changes the text and clicks the button, it invokes the service, uses the _id, and updates that col 'item' in the correct row.
Let me know if that helps and if you have success 
Note: If you have more than one 'item' its best to use listview, and place a invisible label and populate the _id in that, so you know what each _id belongs too.