You can add and remove using either the spreadsheet import (bulk edit) or by editing each variant individually.
Spreadsheet Import (bulk edit)
Navigate to Settings > Locations and click on Import/Export Stock Location Data.
Select the location you want to edit. Click "Download Import Template for <Your Selected Location> (.csv)" to download a spreadsheet of all of your variants and their existing bin locations (if any) in that location.
Open up the file on a spreadsheet program. Fill in the bin location for each of the listed items.
You can input any alphanumerical value into the bin location column. Once you're done, save the file.
Go back to your TradeGecko account. Select "Choose File" and select the file you've just edited. Click "Upload File".
Adding Individually
Go to the Inventory page and select the product and variant you would like to add a bin location to.
Click on the variant. A popup will appear, navigate to the Stock Locations tab.
If you have multiple locations, choose the location row which holds the variant. Change the Bin Location column on that row. Once you're done, proceed by clicking the Save & Close button.
You can also remove an existing Bin Location by emptying the input field and proceeding then saving the changes.