Using these credentials to access accounts you do not own is illegal under most computer crime laws, such as the CFAA in the US.
If you find your own credentials or data associated with such a list: 300k UHQ.txt
These lists are often compiled from multiple data breaches or via "stealer" malware. Using these credentials to access accounts you do
Activate Multi-Factor Authentication (MFA) on all possible accounts to prevent access even if your password is known. 300k UHQ.txt
Immediately update passwords for sensitive accounts, ensuring you use a unique password for every site.
Use services like Have I Been Pwned to see if your email has been part of a known breach.