Complex sentence: The building of the museum was built already in 1971 to celebrate what would have been the 100th birthday of Lenin and up until 1991 it served as the Museum of Red Latvian Riflemen.
Split into simpler sentences: The building of the museum was built already in 1971 to celebrate what would have been the 100th birthday of Lenin. Up until 1991 it served as the Museum of Red Latvian Riflemen.