You can make changes to the translation, yes, but it’s not as simple as editing the text inside the fi.po file (or any PO file, for that matter). That’s because WordPress actually uses another file, with an .MO extension, to render the translations. That extension is not meant to be read by humans, so you wouldn’t be able to just open and translate it. It is compiled from a .PO file.
Anyway, it may seem complicated, but the workflow is actually simple:
1) open you fi.po file in a program which is meant for translating (not just text-editing), like Poedit. It’s cross-platform, so it doesn’t matter what OS you’re using;
2) When you save the changes on the .PO file, it’ll automatically compile the .MO for you, so you just have to take both files and overwrite the original ones for your changes to take effect.
Keep in mind that if you’re changing translations from WordPress’ official translations, or from third-party plug-ins, you might lose all your changes when you update.
Here’s a bit more background on translating WordPress. And if you need help with configuring Poedit, you be sure to read this.