NOTE: If you want to update weights in bulk, please follow this guide.
To update a singular product's weight in Magento is very simple.
Firstly, in the top bar, hover over 'Catalog' and then select 'Manage Products'.
From there, select the product you want to change the weight for.
On the next page, there is a section for the weight of the product.
If you need any help with this, or any other problem. Please get into contact with our support team.