Whether or not Google Bot gives extra bonus points to websites with frequently updated content has been a topic of conversation for many years. One thing that has been mentioned by many is that the answer depends on the industry you are in. For instance, the search engines expect frequent daily changes if you are a news agency than if you own a hair salon. So, first and foremost, you need to think about your industry and if they are a lot of changes to be expected or not.
Regardless of your company, you must agree that providing fresh up-to-date or new content to your website visitors can only be good for them and your ranking. If you already have strong well written content, then focus on adding more pages or more text of existing pages. You may also add optimized images which can also be considered content. Make sure each page is focusing on a specific keyword or a keyword term. Never try to optimize pages (including your homepage) for every keywords in your business. Have a focused approach. This will always render better SEO results for the ranking of your website.
Now, that you have to (to keep your ranking up) or not, creating new content is time consuming. You may hire an expert. If you do, make sure you give them the keywords you are targeting and make sure that same content is not used or sold to others in your industry. Duplicate content on the web continues to be a negative for optimization. If you are trying to do it in house, then again remember the focused approach in terms of keyword empowered content. It needs to be relevant to your business of course but also readable to actual humans. Never write content just for search engines. That is not good marketing and can backfire in terms of your ranking.
If most cases, we recommend our clients to add a new page once a month. If they have a blog, we recommend two posts a month. Remember that it is better to have 12 blog posts a year 3 paragraphs long than 1 single post that is 36 paragraphs long.
So, yes, edit your website content from time to time and add more pages and images so that your website stays relevant and interesting for both your prospective clients and search engines like Google.