how to make or edit alternate text of objects in a pdf file?

I am assuming you have a copy of Acrobat.

Open the "Touchup Reading order tool". Tools->Accessibility->Touchup Reading order

RIgh click your object and choose the option "Edit alternate text"

And, an alternate approach —
Open the Tags panel. Select the <Figure> tag. Select Properties off the option menu.
Enter desired Alternate Text & tab out of the field to "set" entry.