Info:
ZBrush version 2021.5.1 focuses on fixes and tweaks to improve the overall ZBrush 2021.5 experience. It is highly recommended for all ZBrush users.

?Subtool saving updated when using SculptHD mode
?Resolved HD Geometry stability issue
?Dynamic Subdivision with Fibermesh no longer causes visual artifacts
?Material Transparency rendering issue resolved
?Dynamic Subdiv causing artifacts when rotating has been updated
?ZBrush To Photoshop CC updated to fix a layer issue
?Scale Master updated to work with Dynamic Thickness and will now detect hidden geometry
?Decimation Master updated
?Axis Rotation updated
?Maya BlendShapes updated
?Maya *.ma ascii file export fixed
?Option Added: When adjusting the slider the Shift key can now round to a whole value.
?Poseable Symmetry returned to ZBrush 2021.1.2 functionality ? works with multiple subdivision levels
?Inflate brush updated to take advantage of Thick Skin
?DemoHead project restored
?Default UI tweaked

If you have ZBrush 2021 installed (any release) it is recommended to simply use the ZUpgrader application in your ZBrush directory.

If this is a fresh install, install 2021.1.2 then updrade to version 2021.5.1, both are included in this release