I have forgotten 2FA password

If you have forgotten your twostep verification password (2FA password) then it depends on did you set e-mail address for password reset.

I have set e-mail

If you have set e-mail then tap on "Forgot password?" in the bottom of login screen. Telegram platform sends recovery code to your e-mail. Type that code in app and set new 2FA password.

I have not set e-mail

It depends on do you have active sessions in your account

I have active session

If you are logged in your account on any device then

Go to "App menu - Settings - Privacy and Security - Two-step Verification".
Tap on "Forgot password?" in the bottom of the screen.
Tap on "RESET" button

Telegram resets your 2FA password in 7 days after this. Password resetting procedure can be cancelled from any device which has active session of your account.

I have no active sessions

In this case you can not get access to your account. Try to remember the password or reset your account (all data will be lost).