Can I change a wasteland explorers equipment?

Without recalling the explorer, no. Sometimes an explorer will change into an outfit they find, or equip a weapon they find, if it's better*, but this is fully automatic. They'll still bring back their previous equipment.

*Better doesn't always mean what you'd choose. E.g. an explorer will swap out their rusty laser (7 damage) for a 6-8 damage sawn-off shotgun they find.

With the new update (not sure the version number) if your dweller found locations during their exploration you can access his weapon and outfit and change into anything you want.