In order to assign more RAM to your Minecraft server, firstly select your Instance and click on Manage:
From there, navigate to the Configuration section:
Memory Limit (MB) will be found under Java or Java and Memory section.
NOTES:
The default value for Memory Limit (MB) is 1028 MB
You can’t set up more memory than your plan provides
We recommend saving at least 512 MB out of provided RAM in order to prevent server crashes and instabilities.
For example, if your VPS has 2 GB of RAM, the maximum setting for Memory Limit should be 1536 MB), in case you need more memory - consider upgrade