Transferring Time Machine data to external SSD drive and using that drive as the start-up disk

Hi

I have a 2017 iMac 21.5 inch. The hard drive is getting slower and slower with beach balling and taking a long time to open apps, etc. I have bought a Lacie 1TB external hard drive and formatted it, but I am unable to transfer data from Time Machine to this SSD. I want to use the external drive as my start-up disk.

Any advice would be appreciated.