Like any website you navigate to on the internet, each grid has its own unique URL (Uniform Resource Locator), commonly termed as its 'address'. If a grid is hypergrid enabled, you can use this address to teleport there, by copy/pasting it into the Map feature of your viewer. Occasionally this fails, usually because the other grid is running a version of the opensim software that is incompatible with the grid you're on, or because the grid you want to get to is offline (perhaps just for a few hours due to maintenance, so don't give up - try another day!)
See 'How to Hypergrid' for tips and tricks about how to do that without smashing your computer from sheer frustration.
Two good sources of HG informationand destinations:
|Sauce, on Aire Mille Flux|
|The Lost Gardens of Apollo, Carima|
|Landing point, CreaNovale|
|Celestial Japan, Discovery Grid|
|Gardenia Town, Winxtropia|