You can modify the basic attack somewhat using traits- speed and number of hits, mostly. If you want to actually swap it for another skill, you need a plugin: there's one included with the default files called "WeaponSkill," or you can go for
Yanfly's Weapon Unleash for some deeper options, like changing Guard as well, or having individual actors / status effects replace one command with another.
To give a bit of background on why this requires a plugin: Actors don't "learn" the Attack or Guard skills. The Attack and Guard commands on the menu are normally hard-coded to use Skill #1 and Skill #2, regardless of what those skills actually do.