Welcome to the Forum voidology !
Here are all important links:
http://lmms.io/forum/viewtopic.php?f=1&t=4740
-A few rules and useful forum instructions
Yes you can do that. You ned to use a non-discrete automation pattern into an automation-trackchange the tempo halfway through the song
https://lmms.io/wiki/index.php?title=Wo ... Automation
If Lmms is difficult for you, Perhaps my LMMS rookie-guide can interest you? Link in signature.