Best Practice dealing with 3rd party certificates to allow SSL connection

Hi ,

I'm not familiar with Fortinet at all. 

I would like to understand what is the best practice when we need to enable a connection 

to a site like Github and such .

Since the certificate copy  produced by Fortinet replacing the original Github  certificate isn't valid 

by the client and we get a SSL error.

So how do we deal with such a situation ? i.e allowing SSL connection to Github ( as an example ).

I would be much obliged if you could answer me as to a completely noob.

Thanks