I've come back to playing recorder, and I'm more experienced in hearing notes now than before. I noticed that the Low C on my Soprano Recorder (All the fingers covering the holes) and it's a bit sharp. I searched how do I fix this, but I didn't find anything.
Before you blame anything on the recorder, make sure it isn't actually your fault. On the low notes, it's imperative that the fingers close all holes very tightly, even a tiny leak will cause them to either rise in pitch or outright overblow.
Also, these notes aren't really suited for high breath pressure (again, because of overblowing risk) and with very low pressure the pitch goes down automatically. Of course the volume goes down too, but that's just the way it is – if you need a powerful C and D, use an alto recorder, not a soprano.
Assuming you have a good instrument with a separate foot joint: Try pulling the foot joint out a bit (start with 1 mm), this will flatten the C. It will also flatten the C# a bit (but not as much), but having a flat C# is probably not as bad as having a sharp C and it could theoretically be adjusted by an instrument maker.
If you have a cheap plastic instrument with a one-piece body: buy a good one.
Breath pressure is the key to good, in-tune, low tones. That said, make sure your fingers cover all the holes without tension. If you think about exhaling slowly but steadily your low long tone production may improve.
If you simply can't ever get it in tune you can extend the bottom of the instrument with a little tiny tube of paper without messing the rest of the notes too much.