how to change a vertical app to become a horizontal app

Depends upon whether the app allows it, some will only show in one orientation - if you don't have rotation lock 'on' then the app will automatically rotate as you turn the iPad if it supports it.

Just turn the iPad a quarter to be horizontal and the apps turn horizontal. If not look at the side switch right upper side and switch it up to unlock the rotation.

You can set the use of that side switch for lock/unlock rotation or for Mute/umute: see Settings->General-> "use sideswitch to:"

Lex

Click on 'rotation lock' in my above post for info about it (if you don't have a padlock icon at the top right of your screen then you don't have it 'on') - depending upon what you've got Settings > General > Use Side Switch To set to, rotation lock is either controlled by the switch on the side of the iPad above the volume control (if 'Use Side Switch To' is set to 'Lock Rotation'), or via Control Centre (swipe up from the bottom edge of the screen).

iPad2: iPad in vertical position, right side just above the volume control