Before doing anything
BACKUP YOUR SITE!
Step 1: Check that your server supports the requirements
Before doing anything you have to clarify that your hosting environment is suitable to host Joomla 3.x. The technical requirements are outlined clearly in this article on Joomla.org
Any good host is able to run Joomla 3.x. Nothing is special in the requirements! In case your host cannot provide the correct environment to support Joomla 3.x: CHANGE HOST
So assuming all is in the clear with the requirements you go to the next step!
Step 2: Check all extensions compatibility with Joomla 3.x!
Once again (and make a note of what you are doing on a piece of paper so you will remember!) before doing anything yet go and check all extensions (templates/components/plugin/modules) and make sure that any of these extensions are Joomla 3.x compatible. For sure when you have installed a Joomla 2.5 template you will need to change this with a Joomla 3.x compatible template. In general you can see in the Joomla Extension Directory pretty clear that an extension is compatible.
If it only shows the 2.5-icon rest assured the extension is not (yet) available for Joomla 3.x! This means that you have to wait or have to find an extension that IS available for Joomla 3.x...
This is what you will have to do with all your extensions!
Make an inventory what is and what is NOT compatible with Joomla 3.x!
When you have written down this we go to Step 3!
Step 3: Update all extensions compatibility with Joomla 3.x!
This is a simple step! You just download and update all your extensions (not Joomla yet!) you have found to be compatible with Joomla 3.0 except your template (!) Only upgrade extensions that show compatibility with Joomla 1.5/2.5/3/x or 2.5/3.x!
BACKUP YOUR SITE!
Step 4: Delete all extensions that are NOT compatible with Joomla 3.x!
Go to Manage Extensions > and choose "components" : delete those components that are not compatible with Joomla 3.x and do the same for successively "modules" and "plugins" Please be careful and do not delete things you have not changed. Do not delete any Joomla stuff for sure! Do NOT delete what you before in Step 3 have upgraded
Disable plugins: Remember plugin, Google Maps plugin
Change template: Joomla default template
Now when you are done we are going to get your site to Joomla 3.x !
Step 5: Update Joomla 2.5 to Joomla 3.x!
So go to the component "Joomla Update" and select the global "options" icon (top right in your screen). Change the settings from Long Term Release (LTS) to STS and save. Now go back to Joomla update and you will see that the 3.x version is available!!!! Click UPDATE and within seconds your site will be on J3.
Welcome @ Exciting All New Joomla 3 Version!!!
So we are going to finish up now!
Step 6: Update remaining extensions
So Now it is the moment to upload your Joomla 3.x template, configure it. Also do that for extensions you have replaced since they had no Joomla 3.x version (refer to Step 2)
BACKUP YOUR NEW JOOMLA 3 SITE!