You can update Product Description for multiple products at once, from admin page.
Search / add / replace in HTML description code. (HTML code is allowed)
It works with an infinite number of products.

You can filter products by the following conditions:
- Product Name
- Product Tags
- Model
- Categories (multiple selection is possible)
- Manufacturers (multiple selection is possible)
- Filters (multiple selection is possible)
- SKU, UPC, EAN, JAN, ISBN, MPN, Location
- Price (base price)
- Discount: Customer Group, Price, Date Start, Date End
- Special: Customer Group, Price, Date Start, Date End
- Reward Points: Points (Number of points needed to buy a product), Customer Group, Reward points
- Tax Class
- Quantity (stock)
- Minimum Quantity (Force a minimum ordered amount)
- Subtract Stock
- Out Of Stock Status
- Requires Shipping
- Date Available
- Date Added, Date Modified
- Status
- Store
- Attribute, Attribute Value
- Option, Option Value

Products for update:
Filtered products will be listed with pagination, so, it works with an infinite number of products.
You can set how many products shown per page (for slower servers or large databases, it is recommended a smaller number of products per page).
You can deselect or reselect certain products from list.

Products Description update:
- Search / add / replace in HTML description code. (HTML code is allowed)
- Find and replace expression.
- Add expression to the begining of description.
- Add expression to the end of description.
- Delete entire old description and add new expression.
- Update to all languages or to one language.

Admin LIVE DEMO
username: demo
password: demo

In instructions.txt file from downloaded archive you will find full instructions about installation and usage.

Related extensions:
- MASS Products Update PACKAGE
- Admin Advanced Products Filter
- Admin better autocomplete
- ADMIN: Add Category Filter and Categories Column in Product List

Also, you can find this extension in MASS Products Update PACKAGE