Disk space is the computer's storage capacity for loading new applications and saving data.
Memory (RAM) is the computer's capability (muscle) to open (run) those stored applications.
Unplug the computer, open the side cover and locate the ram:
Rock one end out of the socket using the board mounted retainer/lever:
Then push the ram back in place. Be sure you push straight and do not flex the chip. The retainer should snap or at least give the feel it has locked down. Some are fairly hard to push down so don't be shy, but don't be careless either...
Generally it's only necessary to do one end of the ram chip, but do all ram chips and before starting the computer, check the opposite retainer to make sure it is fully locked down as well.