1. config_inc.php has additional data so add it from the previous version.
2. default admin : administrator pass : root
3. Select MySQLi as database type.
4. check edit in all aspect if there is error change where change is required 
   mantis less than 1.3.0 having default "mantis_" prefix for table
5. PHP REQUIREMENT LINK :  https://www.mantisbt.org/docs/master/en-US/Admin_Guide/html/admin.install.requirements.software.html
6. Language : if language changed from dashboard it changes for current user only and other user's language remains English. To change language for all user need to add variable in config http://www.mantisbt.org/docs/master/en-US/Admin_Guide/html-desktop/#admin.config.language
7. Keep $g_db_table_plugin_prefix and $g_db_table_suffix value to "mantis" only in manual installation to avoid confusion in table names.
8. We have asked our users to add the [[softurl]] post completing the upgrade process, check in next version if it's required or not.(Not required, checked in 2.26.3)