How to improve working memory in adults?
IQ is not the same as working memory (either visual or non-visual). IQ is deprecated in favor of the term general intelligence (g).

Education improves g, but not by much; only about one point per year of education (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4445388/), where about 15 points are a standard deviation.

Many things affect general intelligence negatively, such as lead exposure or inadequate nutrition. However, once basic good conditions are experienced, cognitive decline is not avoidable. The good news is that while g decreases in older adults, semantic memory (knowledge) can continue increasing, such that real-world performance on many tasks need not suffer.

Beware a huge range of companies and hucksters promoting activities to slow cognitive decline: to my knowledge, none of them have much validation nor a consensus of scientists without conflicts of interest (e.g., financial).