You should update both providence and pawtucket to the dev/php8 branch at the same time. This version also supports php7.4, so you can upgrade the application first, then upgrade PHP8 (up to v 8.2)
To upgrade, first Backup your database.
Then run git checkout dev/php8
to bring in the updated code (assuming you used git to install develop branch 1.8).
You might want to make sure you haven't made any changes to core code that will be overwritten doing this. git status
will tell you any files that have been modified. Assuming nothing shows here, then you are safe to use the checkout command, git will usually not let you checkout if there are conflicts like this.
Go to the site page and it'll tell you to click a button to perform migrations. This takes you from revision 180 to 195.
Your site will most likely work at this stage, but there might be complaints about some vendor libraries.
To update vendor libraries you'll need to delete the composer.lock file and run composer update
Some other tasks you might need to do are.