Fix a WordPress website

After the php version has been upgraded by the host, my website has stopped working and I get some errors. I fixed some but now I getting this error when trying to access the wordpress admin page:

Warning: Trying to access array offset on value of type bool in /customers/1/c/e/pikykwiky.com/httpd.www/wp-includes/theme.php on line 2246 Warning: Cannot modify header information – headers already sent by (output started at /customers/1/c/e/pikykwiky.com/httpd.www/wp-includes/theme.php:2246) in /customers/1/c/e/pikykwiky.com/httpd.www/wp-includes/pluggable.php on line 1212

Also for the website itself, I can see the header and footer but for any internal webpage I see the error 404.
The aim of this project is to fix the error or maybe redeploy the wordpress theme.