In general, when starting a side income blog, you will find search engines like Google favour updated content, as it shows your site is active and providing accurate information to readers, building trust.
When spending time making a post and it can benefit from an update, I think it’s likely a good move and a positive return on investment (ROI) to give it an update, as the resources used to make it will be better utilised doing that, rather than creating another post. It also expands on a topic and provides accurate information to your audience, improving your authority on a topic.
New information, statistics and data can be the things you target to change when updating content, but I think simply updating the quality of posts in general where possible is also important.
Evergreen content is content that remains accurate long after its publication but still has SEO benefits when updated with new information, images and links.
I think content that you’ve spent a long-time making appealing and is somewhat perfected, can be at a “pinnacle” and so is not easy to make better without restructuring it and so is not a good ROI to update and your time is better spent focussing on areas of your blog that need improvement, but if a possible improvement is noticed, it’s a good idea to update this type of content.