Membership: change text when creating login account

Contribution page for the membership application of a non-profit. When the user joins, a user account is created. The following 2 messages that need the english cleaned up.
-> Please enter a user name to create an account. If you already have an account, please login before completing this form.
->Your preferred username; punctuation is not allowed except for periods, hyphens, and underscores.
Change to:
-> Please enter a Username to create an account. If you already have an account please login before completing this form.
->Punctuation is not allowed in a Username with the exception of periods, hyphens and underscores.