It seems electoral college debate comes back every election these days. What would be needed to replace/remove this system? An amendment? ratification? Is it even possible?
The election of the president (and VP) is a function most directly of Article II Section 1 Clauses 2 - 4 of The Constitution and the 12th and 23rd Amendments. Sections 2-3 of the 14th Amendment would probably have to be rewritten, since they also refer to there being "electors". A single constitutional amendment would suffice (and nothing less than an amendment would): the wording would of course depend on what system you wanted to replace it with. In doing this, it should be decided how to deal with the fact that electors cannot vote for both a president and vice-president from the elector's state (per the 12th Amendment): that might indicate an intent to prevent the president and vice-president from being from the same state, or it might mean that the intent was that if that happens, then that state's electors can't vote for their two favorite sons. In repealing the 12th, you can decide what you want now.
There is a proposal to effectively nullify the Electoral College, the Popular Vote Compact, which has been enacted in a few states. The basic idea is to make it a statutory requirement at the state level that all electors must vote for the winner of the popular vote at the national level, regardless of the outcome in the particular state. The idea is that when enough states agree to the scheme such that they hold a majority of electors (which can change over time, so to be stable you need more than a simple majority), then they vow to vote for the winner of the national election. However, this also needs to be backed up with more compulsory faithful voting of electors, since compact or no compact, only 1 state seems to be able to actually prevent an elector from voting however he wants.