Remove the cleaning rod if your rpk has one, then on the right side of the rear sight block, the is the locking lever move it to the up position may be stiff if you have not done this.
Then just pull away the upper portion of the foregrip with the gas tube. then on the bottom portion of the handguard, there is the metal retaining piece with a friction lever move to the up position then pull the retaining collar away down the barrel, then simply pull the lower handguard away.