I am using adxl345 of elegoo and i have connected it to the DAQ and i am getting the values in voltage. How can i convert that voltage to acceleration? Any suggestions

Well, you have probably set up to have the device return data on some scale and you are not using a library that has already scaled your data. Lets say dataA comes to you, via SPI or I2C in two 8 bit segments with a plus / minus range., That will give you, give or take, about 65K of position information or 32K of positive information and -32K of negative info. Lets say you selected a scale of 4g. The number from the device is a sample of a 4g scale in A:D units. If your number from the device is positive, you'd multiply the reading from the device by (4/32K) to get your actual thingy. I am sure you can figure out what to do for a negative number. There are a few exceptions and rule changes but for now, I hope that helps.

If you find all your data is positive then look to your MSB uint16_t, consider changing that to a int16_t.

The ADXL345 data sheet explains how the output corresponds to accelerations.

We don't have a clue what "the DAQ" does, though, because you forgot to post the details. See the "How to use this forum" post.