Can someone tell me how to change part of a field in a table?

I want to change a phrase in description and url fields from "http://abaris.xxx.xxx/pages/...." to http://leto.xxx.xxx/pages/..." I only want to change wherever there is "abaris" to "leto" in SQL Monitor. Basically I am looking for an sql command to do that. Can someone tell me how to phrase it?

Thanks

Banana