There's no way you can change this manually. Anytime your reputation is under 10000, it will show the actual number without modification. Once you hit 10000, it will start rounding the number to the nearest multiple of 100, creating numbers like 10k, 10.1k, 10.9k, etc. Once you hit 100000, it will start rounding to the nearest thousand, creating numbers like 100k.
These alternate values are only displayed in the user box attached to questions and answers and the network-wide flair image (which always abbreviates for the thousands, to make room for the badges). The profile page and per-site flair images always show the full reputation amount.