Wordpress site with custom database needs updating and fixing

We have a Wordpress site with lots of broken links that need to be fixed. The site also needs updating to https and PHP 7.4. There is some custom functionality and database from Brightminded based on a Responsive template that needs to be preserved. The database needs to be checked and as it needs to be 'updated' - not in its content but somewhere the functionality / links are not working properly - lots of 404 messages. Site can be found at www.responsible-retrofit.org. Note that there may be two databases on the site - the Guidance Wheel is working fine (http://www.responsible-retrofit.org/greenwheel/) and does not need to be touched. It is the other 'side' of the site - the Knowledge Centre that has the issues.