DIAGNOSIS Steam escaping from the lid seal means your sealing ring isn't doing its job. It's either dirty, damaged, or not seated correctly in its channel.
FIX 1) Turn the cooker off and carefully move the steam release to Venting. Let it depressurize completely. 2) Once safe, open the lid. 3) Remove the silicone sealing ring from the underside of the lid. Stretch it, inspect it for tears or permanent kinks, and wash it in warm soapy water. 4) Dry it thoroughly. 5) This is the key part: When putting it back, make sure it's seated evenly all the way around the groove. Run your finger around the entire circle to check for any spots popping out.
IF THAT DIDN'T WORK Your ring might be 'flattened' from heat and use. Try flipping it over (top to bottom). If that stops the leak, it's time for a new ring. They're cheap and should be replaced every 12-18 months with heavy use.
HEADS UP A little initial 'puff' of steam from the float valve as it pops up is normal. Continuous hissing from the rim is not. Also, never overfill past the 'Max' line—that can force food particles into the seal.