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U1TCTL &= ~TXEPT;

what does the "&= ~" sign mean?

The '&=' is called the assignment operator.
For eg.the expression
x=x+2
can also be written as x+=2 using the assignment operator.

Thus U1TCTL &= ~TXEPT; means
U1TCTL = U1TCTL & (~TXEPT)
ie. U1TCTL is equal to the bitwise AND of U1TCTL with the complement (~) of TXEPT.

Also regarding assignment operators do note that expressions like x*=y+2 evaluates to
x=x*(y+2) and not x=x*y+2

i have got it
thanks