Update small PHP website from 5.3 to 5.6

I run a small business with 3 people and have to report sales from two main sources to our customers. I have a website with about 500 users. They use the site to log in, check a table of data and then log out. There is a separate table for each year from 2010 to 2020. I built the site myself with some basic PHP and html 10+ years ago and it now needs updating to PHP 5.6. It has a had some small cosmetic changes along the way (by me) but I am not a programmer and I managed to assemble the PHP more by luck than judgement and some help from Larry Ullman's book! Each year's table is a separate page accessed from an index page with the years listed. Each table has a row for each month. I update the table twice a month by importing a csv file with new data. I don't want the appearance of the site changing at this stage. We supply the user with the username and password so there is no registration. I'd like a comptent person (i.e. someone not like me!) to check it over and then give me a table that works with PHP 5.6. If all goes well maybe we'll look at adding some more bells and whistles later to make the site a bit more contemporary. The files I have now are [login to view URL], [login to view URL], [login to view URL], [login to view URL], [login to view URL], the annual php files and some headers and footer files and a .css file. Many thanks