This sounds like you might have a keyword cannibalization issue.
Search engines will always rank the page that they consider the most relevant page, however, this might not be the page you consider to be most relevant or your conversion page.
You can adjust this by one of several methods, depending on the exact situation and circumstances
1) Page Consolidation
2) Adjusting Internal Linking Structure
3) Breadcrumb navigation and page index schema
4) External links
1 - Page Consolidation
Page consolidation is where you put all of the relevant content from all of your pages into the one main page that is ranking. This page will then be the authoritative page on your site and will likely rank well for your main term and a plethora of long-tail keywords. This page could and possibly should also become a content hub on your site if appropriate.
2 - Adjusting Internal Linking Structure
Sometimes pages rank well compared to other more relevant pages on your site just due to how they are linked internally on your site. If the page that is ranking is linked from a top-level main navigation page then it might have a disproportionate amount of internal links and therefore link equity pointing to it.
You can easily adjust this by either modifying your navigation structure and or adding appropriate in content links and anchors within your site pages, therefore further highlighting and identifying what it is relevant to.
3 - Breadcrumb navigation and page index schema
Related to the internal linking and navigation structure, adding bread crumb navigation and site index schema can help to identify where the page sits in your site hierarchy and can help to make sure that main content pages are shown appropriately in search results.
4 - External links
Finally, getting links to the page you want to rank higher with relevant anchor text from relevant sites will also help the page you consider most appropriate to rank higher.
When this situation happens, it is important to take a step back and consider why the search engine thinks this is the most relevant page. There is usually some insight to be gained here even if you do the above steps and adjust which page ranks higher for your target keyword.
It is also important to think more holistically than just focussing on one keyword. What other keywords and more importantly traffic, conversions and usability do the two different pages have? Make sure that you are not about to sacrifice a good high converting page just to rank higher for one keyword.