In order to open the buy menu, there is two options.
Type !buy in chat, or press the Show Scoreboard key (TAB by default) and Reload (R by default) at the same time.
Then, in order to select sections, one must press the corresponding number to the left of each section.
One can also type !qbuy # # <#> [#] to quickly buy an upgrade or many of an upgrade at once. More info here.
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