Typically you need more memory (RAM+swap) for an update than you do for normal service.
If you have no space for swap, it’s because your forum data (database+uploads) is using the space you need.
It’s just possible that you could follow the strategy
- never update
or the strategy
- migrate your data each time to a fresh up to date installation
But I’ve done my own struggling with a minimal sized instance, using my many decades of sysadmin experience. Eventually, it was preferable to use a bigger machine. In fact, very preferable, because the bigger machine was cheaper. This is because different providers charge different amounts - I moved from Digital Ocean to Hetzner.