Two update your accounts name in Tithe.ly ChMS, first head to the Settings area.
From here, there are two Settings pages that will need to be updated.
Under Settings > Account you can set the account name there. This updates the name that Tithe.ly ChMS has on record for your account, and is what gets used for the %church_name% placeholder in a number of emails.
Under Settings > Layouts each of your layouts will have a different name that you can give. This affects that name that your members will see when logging in.