yeah it has the same plug,so theres no issue with the amps,from 4.74 to 6?

No problem. The laptop will take as much as it needs.

My toshiba use a 19v 3.74a adapter.It crapped out,I have a 19v 6a adapter,can I use that?

If your 19V~6A adapter has same plug type you can use it.

I am looking at buying a new power adapter for my acer aspire laptop. It is giving me 2 options, the fixed U-tip or the fixed C-tip. What is the difference between the 2

What is your laptop model number? Acer Aspire ###?

Thanks

I need a new charger for my laptop. It is 19v charger. On the other hand, I have some 15v charger from my old toshibas. Is there any way to mod my 15v charger into 19v charger?

You cannot do that. Laptop AC adapter not repairable and usually they are sealed units. Just buy a new adapter.
I wouldn’t use the laptop with a 15V adapter. You can damage the motherboard.

I have a 19v-3.95A adapter. Can i use 19v-3.42A?

Most likely the laptop will work with 19v-3.42A. It might take longer to charge the battery but it will work.
I wouldn’t recommend using it for a long time. It’s better to use the same adapter as the original one.
Also, do not buy cheap third party and compatible adapters. Buy the original adapter.
You can buy genuine 19v-3.95A adapter on eBay for about $15-20.