PHP upgrade - 5.6.40 to 7+ - easy?



Yes, i´ll come out and say it right away. Im totally lost in regards to coding... but as im an editor-in-chief at a swedish gamingreviewsite, i just cant seem to find anybody that can help me, or atleast give me advice...

The thing is this...
Our site isnt just functioning good, sure the caching thru WP rocket works, but all them pages thats not cached have sucha long requesttime that they many times dont work for readers. Which takes me to the potential issue at hand...

The PHP isnt updated for a loong time - its 5.6.40, and i guess should work for the site, but it just have issues all the time. We havent had anyone abled enough to look at it for ages, and i wanted therefore to get advice from people who know their stuff - i.e this forum.

First some facts about our site; this is whats its run with:
MySQL, latest Wordpress version 5.4, the theme Soledad, PHP 5.6.40

1. I know how to execute and install a new PHP. But how safe am i to do it? I mean, would you say its safe to do, or should i not attempt it?
2. Im using Query monitor, which points to lots of PHP errors - for example trouble with "Constant WP_MEMORY_LIMIT already defined". Is it possible to hire a person to fix these problems, and would it realistically help the site become faster?

Well theres my initial questions... now i come from a place of lackluster knowledge in regards to PHP, so i hope you can help none-the-less, im really exhausted in that i have tried to find a way to fix the site - i even tried to learn PHP, but it was unmountable, as the PHP knowledge required are years away for me, and the site need help now.

I dont know if its okey to post links to our own sites, but our site is

In any way...

My hopes are atleast that i can get advice, and perhaps (if someones willing) help. No matter off course very appreciated for atleast answers for the two questions.

You're better looking at upgrading through using advise from a Wordpress forum as they will have had the same problems that you will get.

PHP 5.6 is years out of date and should be upgraded, also upgrade MySQL and Wordpress.

I would suggest if you can to set up a test platform which is not connected to the internet to do a test run to iron out any issues.