Use the setprecedence command to set precedence of an operator. Use the declareprefix command to set a specific default left argument for an operator; one can also declare a new unary operator with declareunary or declare an operator implicitly via the definition or proof commands as a unary operator, in which case it is automatically assigned a default left argument of defaultprefix.
The showprecs command will display all precedences, including the formally redundant left/right associativity information.
Declaration display commands will show default left arguments for operators which have them.