i think all one can know must come from personal knowledge. what one believes is only the most likely possibility based on the evidence or reasoning that person can muster up about it-and one person will accept certain evidence that another wont. what one believes can change over time, whereas what one knows, (not what one thinks one knows, but that which one comes to know by personal experience and knowledge), is not likely to change.
the only way possible to change a belief intentionally is to soberly re-examine and possibly discredit or debunk the reasons or evidence behind it and perhaps find more to add to it. one cannot choose to believe something that contradicts one's own reasoning power or evidence in one's hand.
unless of course one is delusional...such as being in love. have you ever run across someone who is in love with a total loser and yet doesnt believe the truth about 'beloved' no matter how many responsible reliable and trustworthy people disclose it? so in a way it is impossible to 'choose' a belief, it is a part of internal consistency. to unbelieve something often requires a huge remapping of central core belief systems.
belief systems are interesting-you can peel them away like the layers of an onion and end up finding out there is nothing at the bottom of any of them.