I have a small river turtle, and I gathered info that they eat vegetables like peas and carrots. I was wondering if frozen ones(once heated and slightly sliced) are in any way bad or different to the turtle? Or can I feed them just as I would fresh ones?
Frozen vegetables should be okay once thawed. There is a slight problem that they lose some of their nutritional value when cooked. Part of the packaging process of frozen vegetables includes blanching them in order to kill off bacteria (blanching is to put some food in a pot of water and bring it up to a boil), so even if you don't cook the vegetables yourself when thawing them (e.g. heating them up in the microwave, steaming them, or boiling them), they've already been cooked before you even bought them.
As long as you're feeding your turtle a balanced diet, it shouldn't make too much of a difference whether they've been cooked. Just note that you might not be able to rely on the vegetables as your sole source of vitamins. Carrots for example will have a lower vitamin C content, so if the only thing you're feeding your turtle that contains vitamin C is carrots, and they're cooked, you might want to look at adding another food or supplement.
If you have the option to get fresh vegetable to share with your turtle I'd definitely suggest that. If you like carrots you can share some peelings off of it, but also the greens from the top that you (probably) wouldn't eat yourself.
What I would avoid though, is canned foods. Because those are usually processed with various preservatives that are bad for pets to eat.