You can replace either one, but it's generally recommended that you replace both at the same time. Replacing just the deck or porch without replacing the railing can be dangerous, because the new deck or porch may not be strong enough to support people walking on it. And if you only replace the railing without replacing the deck or porch, then it's likely that the new railing will not be securely attached to the house and could come loose over time.

It depends on the condition of your deck or porch. If it's in good condition and you just want to give it a new coat of paint or stain, then that's definitely an option. However, if your deck or porch is rotting, crumbling, or otherwise in need of repair, then you'll need to replace it. Replacing a deck or porch can be a big job, but it's definitely worth doing if your current structure is in bad shape. Start by checking out some online guides on how to replace a deck or porch, then get started on your project!