Turning MTB to hybrid
You obviously need a tire of the correct diameter for a &quot;26 inch&quot; wheel - These may be marked 26&quot; or ISO / ETRTO 559.
Generally you'll want a tire narrower than a MTB tire for on-road use. Narrower tires running a higher pressure have less rolling resistance.
How much narrower is up to you depending on the roads or trails you want to ride on, but you are limited by the width of the rim. See What is the maximum or minimum tire width I can fit on my bicycle.