How do you replace belt tensioner on a 200 ford ranger 3.0 liter engine looks like a very tight fit.

See if this link helps:

https://www.askmehelpdesk.com/cars-trucks/faq-how-troubleshoot-repair-maintain-hondas-selected-other-vehicles-46563-5.html#post578394

Approach the problem by assessing what can be temporarily removed or moved aside to improve access; e.g. power steering reservoir or antifreeze reservoir (degasser).