The concept of the colonization of knowledge (technically, the colonization of the lifeworld) comes out of Jurgen Habermas' 1992 "Faktizität und Geltung" ("Between Facts and Norms"). It's an interesting work, but heavy reading. The basic idea rests on a distinction between the kind of language and communication used in the 'lifeworld' — the rich, organic, emotion and insight laden communication that people use in their daily lives — and the restricted and highly structured modes of communication used in 'systems' like science, politics, law, and economics. The restricted language of systems is useful because it makes those systems regulated and efficient, but the problem is that these restricted languages start taking over (colonizing) the lifeworld, making 'normal' communication less rich and vibrant.
An example: Rightists in the US have taken to calling undocumented immigrants 'illegals,' pulling that term out of the restrictive language of the legal system. The effect is to strip undocumented immigrants of all of their human characteristics and evaluate them on a single, monolithic dimension, destroying any possibility of discussing them as human individuals with organic variety and moral standing. The more that these Rightists can push that term 'illegals' into conventional discourse, the more that they colonize that aspect of the lifeworld and dominate the way we communicate about the issue.
Decolonization, then, would merely mean reversing that process: pushing systemic language out of dominant roles and restoring a richer, more organic mode of communication. That can be as simple as pushing back against moronic "three out of four doctors recommend..." advertising (which illicitly imports fake scientific authority), or as complex as trying to disentangle deeply rooted capitalistic assumptions from discussions of gender. I haven't dug into recent work on the issue, so I can't really give details, but hopefully this will give you some insight into the basics.