Update Joomla version - a few concerns

Hello, for security reasons I need to update the my Joomla! version from 2.5 to the latest available. However, I am concern about three things: first, I have installed in my site the OSPT3Map template, can I update to the latest version of Joomla! and be able to continue to use Version 1.0 of OSPT3Map template? second, I made a few changes to the look of the template which I do not want to loose when I update Joomla! Is there anything I must do before updating Joomla! in order not to loose the changes I made to the template?
third, and last, if I may update Joomla! what are the steps I should follow?

Thank you very much in advance for your kind response.

Hi Heidy,
We are not sure if you upgrade Joomla, the T3 plugin of OSP T3Map will work properly. So we advise you to install security extension instead of upgrading Joomla. If you still need to have latest Joomla version, please use OSP Real Estate template instead of OSP T3 Map.
Thanks
Dam