Ak, 

This is not the first time you drop a question with no information at all.

Did you search the Forge?

https://www.outsystems.com/forums/Search.aspx?page=1&q=Email+Forget+Password&scat=forums

Even if you google this you already get good suggestion about the steps you need to implement:
https://www.google.com/search?q=how+to+implement+a+forget+password+flow

What did you try already? I mean you are a developer, you must have thoughts about how to do this.

Can you share what you have so far? What problems did you encounter?

Regards,

Daniel

One possibility is : 

Create new entity to save the request to "forget password" where you kept a code , normally  generated rundomly , and the requester identification. 

Create a way to send a message to an alternative contact , can be another email, or a phone number or a letter by normal mail to the user physical address or any other  ,  with the code saved in the above entity.  

And also the UI associated , the screens to make the request, and the to enter the code that is going to give access to the normal "new password" screen. 

Hope this give you some ideas. 

Regards

Graça

You should have a token in DB with expirationDate, send an email to the user with link with token, when user click on link you validate the token and proceed with change password.