To install an apk, click on it, and it will either begin installing automatically or ask you what program you would like to open it with. Most devices already have a default package installer, however if yours does not then grab one from the play store.
If you're getting a problem parsing the package error, this may mean the apk is corrupted, or your device is not running the correct version.
If you have no device updates available and you are stuck at a version lower than 4.4, you can attempt to manually upgrade- however do so at your own risk! Also, please take caution when downloading and installing apk's.
Music: Pokemon Go Walking Music