There is no "tool" to upgrade, you can just use visual studio to upgrade your nuget version from 7.5 to 8. However it would make senses to upgrade to a more recent versions, because Chrome updates have broken the UI several times over the year.
We had 7.19.4 to fix chrome issues long ago, but updates after that might have broken it again in a different way
Looks like its a bug in Episerver 7.5 edit mode not opening.
Can Episerver 8 supports web forms ?
And please provide tool to upgrade from Episerver 7.5 to 8.