Yes, there are safe ways but 5.0ph is way too acidic.. sounds like it's completely lacking gh and kh buffers. What is the gh and kh of the water and what is the source of your water? It sounds like you're using RO water which is not okay for anything aquatic..
Yes, but they aren't controllable. For instance, you can add Texas Holey Rock, but that will raise it quickly and dangerously to a pH of around 8. Honestly, your betta will be just fine with a pH of 5. :)
Now I know why my PH is hovering around 8. Its been bugging me for a while as I know the granite in the tank is inert and the PH from the tap is only 7.4. Never knew the Texas holey rock raised the PH like that. Sucks cuz its a beautiful rock but looks like I may just take it out before my sorority moves into the tank. I wish the store had of label it as a PH raiser. Oh well you live you learn right.
Is your tank cycled? That's the only reason you would need to raise the pH as beneficial bacteria doesn't do well in extremely acidic conditions. It can crash your cycle if your pH dips down below 7 (usually once you get around 6 it will start to have difficulty processing ammonia).
One way of slowly buffering up your water is to add some crushed coral to your filter media. This will increase the KH of your water and raise the pH. If you add only a small amount at first you can see how much is needed to maintain your desired pH.
I have to agree that 5 is fairly low. Sounds like your water is either very soft or you have something in there causing the pH to drop.