Please use with caution!

Finding the keyword:

grep -rs keyword

-r - recursively

-s - ignore error messages

Search is case-sensitive - use -i to deactivate

Replacing it:

find . -type f -exec sed -i 's/what/helloo/g' {} ';'

is case-sensitive

Bonus: rename only files in the current directory use the -maxdepth 1 option like find . -maxdepth 1 -type f [...].