And if you only have a single instance to replace, you can use ^old^new^
$ echo "hello hello hello world" hello hello hello world $ ^hello^bonjour^ echo "bonjour hello hello world" bonjour hello hello world
$ echo "hello hello hello world" hello hello hello world $ ^hello^bonjour^ echo "bonjour hello hello world" bonjour hello hello world
Why? The last command is more general, you can just drop the g from the command I wrote to only replace the first occurrence. Besides, :[g]s/old/new should already be familiar to anyone who has used vim, so that is almost like having to learn zero new things.
^-t