On the campaign row, click the Bid Strategy indicator (or open View details and click the strategy). In the modal, choose the New Strategy from: Maximize Conversions, Maximize Conversion Value, Target CPA, Target ROAS, Maximize Clicks (Target Spend), Impression Share, Manual CPC or Optimized CPC. For Maximize Conversions and Target CPA, you can enter a Target CPA in your account currency. If you leave it empty, it's plain Maximize Conversions with no target. For Maximize Conversion Value and Target ROAS, enter the ROAS as a decimal number (for example, 4.0 equals 400%); empty means no ROAS target. The change applies immediately, but note that switching strategies restarts the campaign's learning on Google, so avoid frequent changes.