There are two ways to export an account register to Excel. Unfortunately, neither of them matches the on-screen register exactly.
One way is to click on the gear at the top right of the register and select More reports then Register report. This is the same as the report at Reports > Banking > Transaction customized to include just the selected account. You can add and remove columns from this report but it does not include a Balance column.
The other way is to select File then Print <account name> or hit Ctrl-P. This includes the Balance column but does not respect the column selections in the on-screen register. The layout depends on whether you have selected one-line or two-line format for the register.