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 and https://github.com/InvoicePlane/InvoicePlane/blob/master/.github/docs/INSTALLATION.md#prerequisites
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.
7) While manual install set SESS_SAVE_PATH='[[softpath]]/storage/framework/sessions', else installer doesn't continue. Confirmed with vendor https://github.com/InvoicePlane/InvoicePlane/issues/1690
8) On MYSQL related servers if this issue occurs while upgrade ie. Incorrect DATE value: '0000-00-00', users need to modify my.cnf as mentioned here
https://github.com/InvoicePlane/InvoicePlane/issues/769
i.e set sql_mode=ONLY_FULL_GROUP_BY,STRICT_TRANS_TABLES,ERROR_FOR_DIVISION_BY_ZERO,NO_ENGINE_SUBSTITUTION,NO_ZERO_IN_DATE and restart MYSQL.