Bash has built-in substitution. For simple tasks it’s easier than piping through sed:

text="hello world"
echo ${text/hello/ohayou}   # prints ohayou world

Be aware that just the first word is changed:

text="hello world hello"
echo ${text/hello/ohayou}   # prints ohayou world hello

To change all instances of a world, prepend the search pattern with another slash:

text="hello world hello"
echo ${text//hello/ohayou}   # prints ohayou world ohayou

Notice there are two slashes before hello, one as the separator and one prepending it.