If you are using version 3.4.5 now, and it is (or was) working properly, then you can upgrade to current version. There are some considerations outside of the software which you need to be aware of, though.
The server must be compatible, and if it's using PHP 7.3 now, your host will need to switch to PHP 7.4 or higher along with the required PHP Extensions, then a manual upgrade can be done.
Please see this Guide we have prepared:
starting in the section titled, "Unsupported Legacy upgrades to Invision Community".
EDIT: I should add, depending on the size of the existing install this is not a trivial job. You need knowledgeable staff on hand to assist if needed, as you will very likely be required to run SQL queries manually, for example.