As an Admin, you will not be able to set the password for an agent but you can trigger a 'password reset' to allow the agent to set a password for oneself.
Please navigate to Admin > Agents > click on the agent's name to be taken to the profile. Use the 'Reset password' option (only verified agent) to send an email to that agent with a link to change the password.
Alternatively, you can use the 'Forgot password?' option on the login page of your support portal which would also send the same email.
If your account has FreshID enabled, Admins/Account Admins cannot trigger password reset email for other agents from inside Freshdesk. However, this would be possible from the FreshID page upon choosing Agent Login > Forgot Password.