Hi Miki,
there is no search and replace feature in AMOS. To be honest, it has never been any high on my todo list. In many languages, pure search and replace does not work anyway because of the language morphology, grammar etc. Therefore, AMOS offers just find feature in the filter, without the replace part.
There is a workaround, though. You can take your language files from download.moodle.org and replace the string you need in some external editor or tool. Then you can upload the modified files back to the AMOS stage and commit them.
If you still think that an in-built feature is a must for AMOS, feel free to create a feature request in the tracker and let us see if there are some votes indicating broader demand for it.