Turing Machine which will double every 'b' in all inputted string
When you encounter $b$ change state to shifting tape right: move to next cell read content and put blank character.
Keep replacing characters until you encounter blank character (end of your tape content).
Put your last character at the end and move head left until you find blank character. Pur $b$ there.
Keep moving right until you find $b$ and repeat.

In your example:
$\color{green}cbcbcb\_$
Move right
$c\color{green}bcbcb\_$
Change state to copy and put blank:
$cb\color{green}\_bcb\_$ switch state to put $c$ and memorize next element
$cb\_\color{green}ccb\_$ switch state to put $b$ and memorize next element
$cb\_c\color{green}bb\_$ switch state to put $c$ and memorize next element
$cb\_cb\color{green}c\_$ switch state to put $b$ and memorize next element
$cb\_cbc\color{green}b\_$ blank encountered, move back to blank
$cb\_cb\color{green}cb\_$
$cb\_c\color{green}bcb\_$
$cb\_\color{green}cbcb\_$
$cb\color{green}\_cbcb\_$ blank encountered put $b$
$cbb\color{green}cbcb\_$ now repeat move right and switch to copy mode when another $b$ is found. When you reach end of the tape then all $b$'s are doubled.