Minecraft 1.13 has a completely new world format to enable the new water changes. If you try to use a world created on Minecraft 1.12.2 and older your world needs to be converted to the new format.
By default this happens while playing on the server. As soon as a chunk gets loaded, mostly because a player approaches the corresponding chunk, it will be converted to the new format. If a chunk is never loaded, it remains in the old format until it is loaded sometime.
This behaviour may cause lag because the server needs extra resources to convert the old chunks next to the normal gameplay.
The Force upgrade option tells the server to convert all chunks while it is starting. This will reduce potential lag caused by the conversion as all chunks are already updated when the server is online and you start playing.
In order to use it shutdown your server and enable Force upgrade on the worlds page, then start your server. The start process may take much longer now as all chunks get upgraded. We recommend to create a backup before using the Force upgrade option to be safe from possible conversion failures.
If you can't see the Force upgrade option it is not available for the version you play.
Please note that servers with very big worlds may not be able to start when Force upgrade is enabled because your server can exceed the start timeout and get considered as crashed. In such a case you can not use the Force upgrade option.
After all chunks got upgraded the Force upgrade option will disable itself. You don't need to enable it again as it makes no sense to convert all chunks again if they are already in the new format.