This article covers updating items in your estimate one item at a time, if you're instead looking to update items in bulk there is another article for that:
1. On your estimate, if you have linked a catalog item, you will see a label showing the linked item.
For more information on linking catalogue items in the first place see this other article.
2. Clicking on the label will show you more details.
3. You can see the item code, pricelist the item came from, date the catalog was last updated, the description of the item and its unit price.
4. If you wish to unlink this item from your estimate select "Remove link to item"
5. If you wish to change the link to a different item, select "Replace item"
6. Once you have selected "Replace item", the Browse Price Lists screen will appear over the top of your estimate, allowing you to search for the item you wish to link
7. Enter the text you wish to search for to bring back a list of items matching your search
8. To select an item to link, simply click on its description
9. or select "cancel" to cancel your search and cancel the replacement of the link