plugin help. editing database – do i use actions?

I have a a series of complicated problems relating to the plugin i’m writing, which is essentially a product database.

1. I would like to edit the product entries within my plugin. I was hoping that I could just use something like ‘admin.php?page=myshop/productedit.php?id=6’
to edit the item but I get any error. (Essentially, I tried to query by product_id (id=6) and edit that way.

Can someone give me a code or site example on how I edit items in my database tables? I would like to be able to do this within a form and then save the updated data back in the table.

Do I use hooks/actions to do this? I tried to look at the way wordpress makes posts editable and saves them but I feel really confused.