Hi
Unfortunately, srx210 memory is not upgradable, just by design. If it was srx100, you could buy a license to increase memory from 512m to 1g (as it always has 1g inside even for a cheaper low-mem version), but for srx210 - you have to replace it to run UTM.
Note that currently you can buy srx210he - high-memory and faster CPU, than it was in old (non-"e") srx210. It goes at the same price as old srx210h.