Text the word PASS to 43430
you will get your correct username and the email address you put down in your account related to the sim
a reset password link is included
it can take a little time to get an answer though sometimes 10 to 15 minutes
if you cant access the email address to reset password then you will need to contact an agent https://giffgaff.com/support/ask/sim_my_limit_increase dont worry about the catagory as agents will read it.in further information explain you cant get to your email.
also include your username.....your full name and address.....your number
other things to put are sim number on the back of the sim
IMEI number dial #06# and a couple of numbers you would normally call.
sounds a lot but it only takes a few minutes.
give email address you want to use now of course
as per normal agents can take up to 24 hours to reply