Firstly, you need to have your recovery key phrase on hand. Then, you need to choose the option of Restore Existing Wallet as shown below.
NB! If you use a hardware wallet (Ledger or Trezor), you need to go through Connect hardware wallet instead. Never enter your Ledger's/Trezor's recovery phrase in Yoroi.
Cardano key phrases can have 15 or 24 words. Also, you can restore a paper wallet. Yoroi-generated phrases are always 15 words long, but yours may consist of 24 words (e.g. you created a wallet in Daedalus). We will go with the 15-word phrase here.
Next, you'll be prompted to input the phrase word by word. Remember that the correct word order is crucial. If you cannot find a word, you can cross-reference your recovery phrase with the list of words that were used to generate your phrase in the BIP39 word list. This method will allow you to check if these have been spelled and recorded correctly.
Next, you need to set your wallet details. The name and password do not have to be the same as the ones used before. If you have forgotten your password, this is the method to create a new one.
When finished, press RESTORE. Your wallet will be completely restored after this point (please allow a moment for Yoroi to synchronize with the blockchain).