By default, mu4e sorts messages by date, in descending order: the most recent messages are shown at the top. In addition, the messages are threaded, i.e., shown in the context of a discussion thread; this also affects the sort order.
The header field used for sorting is indicated by “V” or “^”22, indicating the sort order (descending or ascending, respectively).
You can change the sort order by clicking the corresponding field with the mouse, or with M-x mu4e-headers-change-sorting (O); note that not all fields can be used for sorting. You can toggle threading on/off using M-x mu4e-headers-toggle-threading or P. For both of these functions, unless you provide a prefix argument (C-u), the current search is updated immediately using the new parameters. You can toggle full-search (Searching) using M-x mu4e-headers-toggle-full-search or Q.
If you want to change the defaults for these settings, you can use the variables
or you can use little graphical triangles; see variable