API EditBox SetFont

To set the font for this object please see, API_FontInstance_SetFont.
Instead of using FontInstance:SetFont use MyEditBoxObject:SetFont. Eg:
 local MyEditBox = CreateFrame('EditBox', 'EditBoxGlobalName' or nil, ParentFrame or nil, EditBoxTemplate or nil)
 MyEditBox:SetFont(FontPath or nil, FontSize or nil, FontExtra or nil)
Please, read API_FontInstance_SetFont for more exact details.
EditBox SetFont