As uSync uses quite a lot of the underlying API of Umbraco, it is susceptible to changes that otherwise make little difference to the user. Changes in the Umbraco API mean that there are different Versions of uSync depending on what version of Umbraco you are using:
|Umbraco Version||uSync Version|
|Umbraco 7.6+||uSync 4.x.x|
|Umbraco 7.4+||uSync 3.x.x.740|
|Umbraco 7.3.x||uSync 3.0.x|
|Umbraco 7.0- 7.2.x||uSync 2.5.x|
|Umbraco 6.2.5+||uSync 2.2.6|
|Umbraco 6.2 - 6.24||uSync 2.2.5|
|Umbraco 6.1.x||uSync 1.6|
|Umbraco 6.0x||No working version|
|Umbraco 4.10+||uSync 1.3.3|
You should install the latest version for your version of Umbraco.
When you Upgrade Umbraco, it is also recommended that you upgrade uSync.
Once you have upgraded Umbraco and uSync you should remove your uSync folder and let uSync create a new sync.
While there is compatibility between versions, the matching and change detection is much more reliable and quicker when the uSync files are from the latest versions of uSync.