If you still have the flash stick (or whatever you used to install OSX), you got two easy ways.
1. Do an install on another stick or external disk. Then copy your data into this new install (either right after the install reboot, or later using Migration Assistant from the Utilities folder), and use Carbon Copy Cloner to clone this install to your original harddisk. Pro: more complicated, but you can test the new install before overwriting the internal disk.
2. Or you simply do the installation on the harddisk again. Depending on the OSX version you either get the choice to preserve (user data, etc) and install; or the data are automagically preserved. Pro: Really easy.