bathroom drains

yes I have a question I need to replace some cast iron drain pipe in my
bathroom . Would you recomend doing it from under the house or go through
the floor as it needs to be replaced also. I have done alot of repairs but this is the first time for this can i replace the cast elbow with PVC thank you
for your help

What size pipe are you talking about, most bathroom sink and tub drains would be galvanized not cast.

the size is 4in

Yes you can replace it with PVC. We don't have enough of information do you have basement or a crawlspace, if it is a crawlspace how big it is?

yes to answer a question from cookie,I have a small crawl space but I
am going to tear out the floor and wall to do it right .and start over.