When a postcard has a defect in condition, it is still collectable. Consider that it may be one of the few remaining postcards like that in existence, particularly if it is quite old. After all, how many 100 year old postcards must there be?
So how to repair a defect in a postcard can best be answered in a word: don't.
Just hold it until a better example comes to your attention, as if that will ever happen.
Pencil prices can be removed carefully. But messages written should remain. They add interest to a postcard.
And in all cases leave the postage stamp on the card. The stamp can add a slight value to a postcard as well as establishing authenticity. Just saying.