I have installed cuda7.5 in my Ubuntu 14.04, but I also want to install cuda7.0, how should I do?

Accelerated Computing CUDA CUDA Setup and Installation
1

In the case that reinstall 7.5, then install 7.0 would be a trouble, so I want to the two version exist at the same time in my Ubuntu 14.04(64bit)

2

CUDA Toolkit supports side-by-side installation.
Please note that the CUDA 7.5 Toolkit requires v352.39 and newer driver.
Please also refer to the following doc “Handle Conflicting Installation Methods”