Backing up your important computer's data is always a good idea when using the computer.
Your personal space and other directories that you create will not be touched without specifying
erase disk or
Installing Ubuntu fresh will not affect a user's personal data and files unless he instructs the installation process to format a drive or partition. The wording in the steps that will do this is
Erase disk and install Ubuntu, and
Format Partition. If you don't choose those to options, the only areas that would be replaced are the ones associated with the new install. It'll list those areas also and advise you that specific data will be
replaced. This includes:
/etc, /lib, /usr, /var, ...
These are systemwide areas, whereas your data should be in your personal area or some other directory that you have created.
To ensure preserve your data during the installation use the following options during installtion:
* Installation type - checkmark Something else
* Install Partition - Ensure Format box is unchecked