There's no setting in InDesign to increase RAM. You'd get more RAM for your computer.
But, exactly what problem are you having with type? Usually, working with type is not RAM intensive.
Short answer: You can’t.
What OS do you use? Is it compatible with your InDesign version?
I have OS X El Capitan 10.11.6 with 24 GB Memory.
Using InDesign 5.5
The difficulty I’m facing is type select panel at top of screen after a few minute goes blank when trying to select a different typeface. The other issue at times is when selecting copy in a document, it disappears as I’m making a copy change.
I felt it was the lack of InDesign running out of RAM.
The fact that it’s even running puts you way ahead of the game.
CS5.5 is unsupported under El Capitan and Sierra. I reached end of life long ago. Many people can even launch it without crashing.
You are, unfortunately, on your own here.