I am using a VPS with Ubuntu 12.04. ISPmanager is installed and PHP version is 5.3.10. How may I update PHP version to 5.3.26?
PHP 5.3.10 is the official package from the Ubuntu repositories. It should be working fairly well and security issues will be corrected by the Ubuntu release team when needed. Upgrading to 5.3.26 will not bring you much difference, but I'll assume you know what you're doing and you have a good reason to want the very latest of PHP 5.3 series.
There are some unofficial Ubuntu repositories that provides newer PHP 5.3 versions. One of them is the Skettler PPA (Personal Package Archive). As said in the name, its a personal archive, so the updates, security corrections and packaging might not be at the same quality level as the ones from the official Ubuntu repositories. I don't know about this Skettler, but the packages may also be of good quality, do your research.
If you wish to use PHP packages coming from this PPA, as explained in the PPA page, you need to add a new package source to your Ubuntu configuration. The source you need to add is:
You'll also need to add the package maintainer PGP key to your keyring so that the package signatures are correctly verified. All the installation process is described in the Read about installing link on the PPA page linked above.
You'll also need to upgrade all the php dependent packages (modules, ...). Check if all the ones you need are existing in this PPA.
This PPA also contains PHP 5.4 packages. But be aware that some PHP applications (ISPManager might be one of them) needs upgrade before being able to run with PHP 5.4. Usually, it's better to stick to official packages, unless you have a good reason to want something else.
There is no simple way. According to the Ubuntu Package Index for 12.04, the PHP version is 5.3.10. So the classic
You need to add an extra repository for more current PHP packages. If PHP 5.4 is OK for you, there's a quite popular blog post with a short how-to on this.
For possible troubles with command 1 see the blog post.
But be advised: Make a backup before! I don't know anything about possible impacts on other web applications you might run or impacts on ISPmanager.