I'll admit, I let my anger at the situation get the better of me. A succession crisis in the US at this very moment should be avoided if at all possible.
But yeah, overall I want Kamala Harris to replace Biden.
There are three mechanisms available:
1) Biden retires in favor of Kamala Harris. It protects the integrity of everyone and can be done smoothly. If Biden has indeed been suffering mental health issues, it is probably the preferred and most likely option.
2) Kamala Harris invokes Section 4 of the 25th Amendment with the assistance of the Cabinet. Alternatively, Congress creates/empowers a body to investigate the mental fitness of the President with consent of Kamala Harris, who would ultimately have to concur with their findings in order to invoke Section 4 of the 25th Amendment
3) The House of Representatives initiates Impeachment Proceedings under "High crimes and misdemeanors". The term "misdemeanors" is not the same as in the Criminal Law usage. One or more prior indiscretions would be being examined under a microscope. Assuming/remaining in office with a known severe issue that impairs judgment could be argued as similar to assuming/remaining in office with a known conflict of interest. Behavior is grounds for impeachment as bringing the Presidency "into contempt, ridicule, and disgrace". This is the option a Republican Majority could attempt if one existed, as it does not require the cooperation of the President or the Vice-President.