This depends on which version of WriteMapper you're using.
On Desktop Versions
On the macOS and Windows desktop versions, you can get to the Preferences window by going to WriteMapper > Preferences in the menu bar. On Windows, hover over the grid button icon in the top-left of the window to reveal the menu bar. In the window that shows, you can navigate the tabs across the top to show different settings you can change.
On Mobile Versions (WriteMapper 2)
On iOS, you can get to the app's Preferences by first opening up any WriteMap file. Use the tutorial WriteMap if you haven't got one handy, or creating a new one works as well. Once you have the WriteMap view open, tap on the more button in the bottom toolbar (), and select "Preferences Menu..." from the menu that pops up, and then choose your desired option.
From there, you should be able to select the setting you wish to change and set it to your desired configuration. No saving is necessary as the setting goes into effect once you've toggle it to a new option. These settings will persist even after you close the app.
Do also note that some settings (like the spellcheck or custom keyboard shortcut) on the desktop versions require a restart of the app before they get applied. Do ensure that you save your work before performing a restart of the app.