The simple answer is "you can't". As a general rule, airline points can't be converted to points in another airlines program - even when they are partners. (There are a few exception, such as the fact you can convert SQ miles to Virgin Australia points, although even that is on hold at the moment).

You may be able to do it via a 3rd program, but the conversation ratio is generally poor. For SQ->UA it looks like you could go via Shangri-La Golden Circle points, as you can convert SQ KrisFlyer miles -> Shangri-La points, and then Shangri-La -> UA MileagePlus miles. The catch is that each 12 SQ miles becomes 1 GC point, which then becomes 1 UA mile, so it's a 12:1 ratio from SQ to UA.