1) Make following changes in .htaccess 
RewriteBase [[relativeurl_]]
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.