Are you asking why you are running low on space? Update is telling you that you need 700MB more space than you have available, right?
Connect your iPad to a Mac or PC with iTunes. Do a backup and restore. That will erase your iPad, download/install most recent 11.4.1, then give you the opportunity to restore from your backup. The full restore usually gives you back some of the space, cleaning out app caches, and other space-taking files...