You can't, UMAX don't support Windows 7 on any of the Astra range of scanners, basically those models are too old:
https://store-y1xcm.mybigcommerce.com/do-you-have-windows-7-or-8-driver-for-my-old-astra-scanner/
If they were going to release a Windows 7 driver for it, they would have done so by now. I wouldn't hold your breath waiting for one.
You could try scanning software called "VueScan" but it's not free. There's is a free trial you can download to see if it works before buying it: http://www.hamrick.com/
My bad. I just checked the compatibility list on the VueScan website. The Astra 7350 is not supported.
You will have to buy a new scanner in that case.