First, make a backup of the files and the database!
(You can do the backup with akeeba backup or with Easy Joomla Backup by Viktor Vogel, for example.)
You'll cover how to restore a Joomla installation after a fatal error!
Or you can create a copy of your site and test it first. (Nobody knows your site and what you have installed)

Then you have to install the latest Joomla version. At this point this should be: Joomla 4.4.14.

Then download the latest version of your template and install it. (but it's probably already 2.0.0)

You then look in the plugins to see whether the behavior – backwards compatibility plugin is activated???
This plugin ensures that your J51 templates and modules work in J5. Basically you check whether there are updates for the J51 modules, they also have to be up to date...

Then you can try the update to J5 (NOT J6!!!!!)...

No guarantee on your other extensions and your entire site. An update like this is always a good way to search for and uninstall certain unused things after backing up your data...