An attribute boost normally increases an attribute modifier’s value by 1. However, if the attribute modifier to which you’re applying an attribute boost is already +4 or higher, instead mark “partial boost” on the character sheet for that attribute. If the attribute already has a partial boost invested in it, increase the modifier by 1 and uncheck the box. At 1st level, a character can never have any attribute modifier that’s higher than +4.
When your character receives an attribute boost, the rules indicate whether it must be applied to a specific attribute modifier, to one of a limited list, or whether it is a “free” attribute boost that can be applied to any attribute modifier of your choice. Dwarves, for example, receive an attribute boost to their Constitution modifier and their Wisdom modifier, as well as one free attribute boost, which can be applied to any other attribute.
When you gain multiple attribute boosts at the same time, you must apply each one to a different modifier. This means you can’t apply a partial boost to an attribute modifier and apply another boost simultaneously to increase it.