Portainer does not have a password reset feature. We will have to you use a password container helper.
1- Firstly, we will stop the Portainer container.
docker container stop portainer
2- We will run the helper using the same bind-mount/volume for the data volume
docker run --rm -v portainer_data:/data portainer/helper-reset-password
The response should be similar to the below.
2020/06/04 00:13:58 Password succesfully updated for user: admin
2020/06/04 00:13:58 Use the following password to login: &_4#\3^5V8vLTd)E"NWiJBs26G*9HPl1
3- Finally, we will restart the Portainer container.
docker container start portainer