There is no upgrade version so you would need to backup your own data, files and folders and then uninstall 1.7.2 completely.
Then install 1.7.7 and re-configure your customizations etc. using your backup data.
There is an alternate method and that is to rename the xampp folder to xampp172 for example but first Stop all XAMPP components and remove any ticks in the Svc boxes of the XAMPP Control Panel.
When you are sure that everything in 1.7.2 is stopped and the services (Svc) are uninstalled then you can install 1.7.7 to the default location keeping the default xampp folder name.
Now you have the best of both worlds 1.7.2 and 1.7.7 with all your data etc. intact ready to progressively configure 1.7.7.
You can only run one XAMPP at a time stopping one before starting the other.
Also in 1.7.2 you would need to run the setup_xampp.bat file to have it reset the paths to the new xampp172 folder name.