sed pattern match problem

Hi all, hoping this is a simple one, tried looking but just can't see the solution

As an example I've got a list of words that all start Ben.....

Bendicks
Benefiber
Ben
Benylin

I need to only change the line Ben with Ben 10, ignoring the other lines.

I tried the following

This works however changes all the rest also, how do you get sed to only focus on a select string pattern?

Many thanks

Maybe try adding a $:

sed -i "s/^Ben$/Ben 10/g"