Uninstall old LMMS from the Uninstaller.exe present along with LMMS. Reinstall new LMMS from lmms.io site.
Warning: avoid using 1.3.0 as your main LMMS because it is unstable and likely to crash if you do something silly (Like hit Ctrl-Right on an empty piano roll). Stick to stable versions, try 1.2.2. Also that 1.3 may not be able to open some songs 1.2 could; for instance I have a song made almost entirely of samples and 1.3 alpha can't open it right.