First, if you don't already have a Roku account, go to roku.com and set one up with your chosen email address.
Then, do a factory reset on your tv to erase all its settings and set it up like it was a new set, and log into your roku account. I have separate Roku devices, not a Roku tv, so I don't know the menu path, but on my devices Factory reset would be under Settings > System
Roku Streaming Stick +, 3810X, Ser YH0059427035, wifi - Samsung UN55ES6100
Roku Streaming Stick, 3600X, Ser 5S56D8240827, wifi - JVC EM37T