Hi loralai
The easiest way to do this is to change the language file for jOpenSim. You find it in your Joomla installation at /path-to-joomla/language/en-GB/en-GB.com_opensim.ini. E.g. the words "Details of your OpenSim Account" can be changed in the line starting with "INWORLD_DETAILS". If there are any other languages installed, of course replace en-GB with the correct language name.
Mind that this file will be overwritten when updating jOpenSim in the future. As of Joomla 1.6+, you also have the possibility to add such lines into /path-to-joomla/language/overrides/en-GB.override.ini. Like this even an update will keep your individual changes in this language (allthough it is always a good idea to make backups before upgrading...)
Greetz
FoTo50
[edit]PS: dont forget to save the language file in "UTF-8 no BOM"[/edit]