A bit of extra detail, on an underpowered machine the interruption could be some tens of minutes. On a fast machine, maybe a few minutes.
Because upgrades can go wrong, you should always start by taking a backup. Preferably for safety download a local copy too.
If the upgrade does go wrong, be ready to ask for help on this forum, with evidence from your logs: the last few hundred lines of output should be enough. Sometimes the first few lines of output.
In case of failure to upgrade, it might be that you can resurrect the previous state of your forum software: