You cannot alter your IQ. It simply cannot be done.
You can ensure you are optimally utilising your brain by giving it ideal conditions, including highly regimented consistent sleep, micro and macronutrients.
IQ tests are a pseudo-science. The only place where they're even remotely relevant is whether a child should skip a class or not.
"Altering" your IQ is easy and same as any other test. Did you get a good sleep last night? Are you hungry? Which time of the day do you take a test in? Is there something in the back of your mind that makes it more difficult for you to focus? Are the questions designed with a different demographic in mind (for example, you're used to right-to-left text while the test is written left-to-right)? Do you like puzzles or do you consider them a hassle? Etc etc.
You're also definitely not as intelligent as a child as you are in old age, meaning that your intelligence absolutely changes during the course of your life. Hence, the decisions you're making today can and do impact your intelligence later in life.
Your IQ does change over the course of your life (your crystalline IQ falls off past a certain age) but you cannot change it.
Dismissing it as pseudoscience is unhelpful. Its the most accurate measure we have, its also wholly reliable for what it is used for. Yes, people who have access to IQ tests can eventually game them, so that always needs to be considered.
The IQ tests and their subtests can also be relevant for indicating possible cognitive conditions like NVLD and autism. For example a person that scores less than 75 on non verbal subtests and over 130 on verbal subtests likely has some condition affecting performance.
You can ensure you are optimally utilising your brain by giving it ideal conditions, including highly regimented consistent sleep, micro and macronutrients.