Upgrading from 3.0.X is usually straight forward especially if you have no modifications. Some 3.0.X modifications added tables and and changes to the database and the normal upgrade process includes removing them. However that is usually inconsequential.
Make sure you have backup of the database and files before proceeding and simply follow the instructions here:
https://www.phpbb.com/support/docs/en/3 ... /upgrade3/
These are the problematic instructions since you do not have access to ACP:
Deactivate all styles except for prosilver
It appears you are using prosilver and this should not be a problem.
Remove all MOD-related changes from the database. The Support Toolkit's Database Cleaner can be used for this
.
This is not absolutely required but is best practice. If you have no mods installed it's irrelevant.
Set British English as the only language pack
If you have set a different language like US English download the language files and upload them before proceeding with upgrade.
“Results! Why, man, I have gotten a lot of results! I have found several thousand things that won’t work.”
Attributed - Thomas Edison