• Open the File Explorer.
• Navigate to C:\Users\<User>\Documents\My Games\Rainbow Six - Siege\UplayID.
UplayID is a long alphanumeric string unique to your Ubisoft account. You will see multiple folders if you have played on more than one account.
Delete the folders and start the game from your account. You should now see a single folder for your account containing a default GameSettings.ini file.
• Right-click the GameSettings.ini file.
• Choose Open with and select Notepad.
You will see a list of game settings.
• Scroll to the bottom of the document.
You will see the following:

[ONLINE]
;DataCenterHint => 
;    default (ping based)
;    eastus
;    centralus
;    southcentralus
;    westus
;    brazilsouth
;    northeurope
;    westeurope
;    southafricanorth
;    eastasia
;    southeastasia
;    australiaeast
;    australiasoutheast
;    japanwest

DataCenterHint=playfab/default

 

All lines with a semicolon in front are a list of the possible datacentre choices. The bottom line (DataCenterHint=playfab/default) is the selection currently set to default, which means the datacentre will be chosen automatically.

• Change default selection to the datacentre of your choosing.

For example: DataCenterHint=playfab/westeurope (West Europe) or DataCenterHint=playfab/centralus (Central US).
• Save the file, then start the game.
• Once in game, press F10 or click the gear icon in the top-right corner.
Below the options, you will see the currently selected datacentre, your ping to the right of it and your NAT type.
• Confirm that your manual selection has been successful and if so, you are all set!

You can change your server at any time by closing the game and changing the values as instructed above.