In order to allocate more RAM to your Minecraft server, first select your Instance and click on Manage:
From there, navigate to the Configuration section:
Then, navigate to the Minecraft section on the left sidebar and select Java and Memory on the top bar. You can change the Memory Limit (MB) here:
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 the 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 an upgrade