where i can download tacos big sur my current one is mac yosmite 2015

Moving from Yosemite to Big Sur is a Major Jump. One may consider moving to High Sierra first. Realize the end game is Big Sur but the High Sierra movie may just help with any Boot ROM updates before move all the way to Big Sur. Understand, Mojave is the last macOS to support 32 bit applications / extensions. Any thing newer like Catalina and Big Sur Do Not support anything 32 bit but Only 64 Bit. Also, may Applications will need updating / upgrading as one moves from one OS to a newer version. Suggest checking with Developers before advancing.

Further, Before going forward to any upGrade, have a Time Machine Backup Before hand.