Hi Sleepy_3.14,
Unfortunately it’s not really effective to try and fix a mattress that was either too soft originally or has softened and lost its comfort or support. There is more about this and some possible options that may partially or temporarily help in some cases in post #4 here but in most cases you are better off with a new mattress.
You’ve probably read this already but just in case … post #1 here has some of the most important information you will need along with some steps and guidelines that can help you make the best possible choice.
Some of the better options or possibilities I’m aware of in the Reno area are listed in post #3 here.
Phoenix