@TokoyamiSenshi ш and щ are two separate letters and can't be replaced by one another. Unlike other consonants which kind of have "hard" and "soft" forms Ш can be only hard (sh) and Щ can be only soft (if you were to pronounce it I guess you could try to make a "shsh" sound, like in rush-ship (the sound which makes sh-sh). If you have problems with this letters, you may try pronounce "Щ" like Byelorussians: shch, it'll still be understandable and close enough.
I do know the word where щ sounds like ш, but it's rare and depends usually on the factor of human mouth just not being able to pronounce щ in that position while the word was just formed by the rules of the language. To explain it I should go a bit futher in how words are formed and it's a bit early for that.
I think I will include to the next lesson more info about it, like the way you pronounce hard and soft versions of the same letter and how you pronounce voiced and voiceless consonants (the thing I mentioned about stressing). It kind of doesn't have that much importance, but you should know those not to get confused by the speech of a native speaker I guess.