mac says its up to date but its not

if your mac shows OS X Yosemite as "up to date", it could mean that it is running the most recent macOS that your mac is compatible with. you can try reading How to get old versions of macOS to check compatibility of a more recent version your mac can run. if it's new enough you may be able to run big sur. you can check compatibility for that by reading How to upgrade to macOS Big Sur.

Please read the following user tip and restate your question/problem in detail: 

Writing an Effective Apple Support Communities Question - Apple Community

Is this your Mac: MacBook Pro (2020 and later). It can't be if you're really running 10.10.5. What model and year is your Mac? In order to to run Big Sur it must be a late 2013 model or newer.

As @ jeffreythefrog outlined - it is up-to-date. There are not more updates for Yosemite. Maybe upGrades and that is a different thing.

The Age of this computer will Govern what newer version of macOS it will Qualify to install. Use About this Mac from the Apple Icon on Desktop - the required Information will be there.

With the above information in-hand - follow this How to get old versions of macOS and verify what version this computer Qualifies to install. Make sure to use Only Safari to commence the download as Others may not work. Safari will open a redirect and open the Apple Store where you can commence the download. If an error message appears, that is probably because one is using another Browser.

Suggest using Go64 to find 32 bit application before attempting the upGrade. Some or many of the Application already installed may required update or upGrades to 64 Bit Versions.

Make a Time Machine Backup Before attempting. Notation - Mojave is the Last version to support 32 bit applications / extensions and Drivers. Catalina & Big Sur support 64 bit and there is no workaround.

Are there any AntiVirus, Disk Cleaner, Optimizers, Un-installers, etc installed which should be removed as per Developers Instructions. They are useless, unneeded, cause havoc and interfere with the normal operation of the OS and may even Corrupt the OS requiring a Reinstallation

As a small addition to what was already said: to know exactly how far your mac can go in terms of upGrading the OS, go to the Apple menu, choose About this Mac, and tell us the model and year.

Only then we can know for sure if you can or cannot upGrade (ie run an OS later than Yosemite, like El Capitan, Sierra and so on). Your 10.10.5 is the final version of Yosemite (so there are no upDates, which would mean newer versions of Yosemite)