1) Make following changes in .htaccess 
RewriteRule . [[relativeurl]]/index.php [L]
It is mentioned in installer and on installation document
2) Please check if new language is added in /application/language then update install.xml
3) Follow the upgrade guide here :
https://wiki.invoiceplane.com/en/1.6/getting-started/updating-ip
4)PHP REQUIREMENT LINK : https://wiki.invoiceplane.com/en/1.6/getting-started/requirements
5) We have kept min PHP version to 8.1 because composer.json states the same. Also we have removed mcrypt and xmlrpc extentions as these are moved to PECL library.
6) Keep the default(recommended) VAT calculation method in manual installation.