There's no way to get those tables to fit at the normal size.
You can either reduce the size, I found that 7.5pt fits, or use three tables.
\documentclass{eusflat2021}
\usepackage{lipsum} % for mock text
\title{\bfseries Ifsa-Eusflat 2021 Bratislava: Instructions for Authors}
\author{Author$^a$ \and $^*$Corresponding Author$^b$ \and Author$^{b,c}$\\
$^a$Department, Faculty, University, Address, \email{[email protected]} \\
$^b$Department, Faculty, University, Address, \email{[email protected]} \\
$^c$Department, Faculty, University, Address, \email{[email protected]}}
\begin{document}
\maketitle
\begin{abstract}
The abstract must be indented 0.7 cm both on left as well as right-hand margins.
Please, do not place or cite tables and figures in the abstract.
{\bfseries Keywords:} Start with capital, Use comma, At least three keywords.
\end{abstract}
\lipsum[1][1-4]
\begin{table}[ht]
\centering
\fontsize{7.5}{8.6}\selectfont
\sbox{0}{%
\begin{tabular}{|c|c|c|c|c|c|} \hline
{\bfseries GTW} & {\bfseries 10} & {\bfseries 15} & {\bfseries 20} & {\bfseries 25} & {\bfseries 30} \\\hline
Entropy IDF & 0.4301& 0.4552&0.4627&\textbf{0.4648}&0.4610 \\
Normal & \textbf{0.4856} & 0.4802 &0.4769&0.4266&0.4181\\
ProbIDF &0.3333&0.3628&0.3717&\textbf{0.3831}&0.3697\\
LDA &\textbf{0.4668}&0.4587&0.4582&0.4482&0.4378 \\
Norm-alt & \textbf{0.5598}&0.5066&0.5077&0.4685&0.4876\\
\hline
\end{tabular}%
}
\usebox{0}
\makebox[\wd0][l]{%
\begin{tabular}{|c|c|c|c|c|c} \hline
{\bfseries GTW} & {\bfseries 35} & {\bfseries 40} & {\bfseries 45} & {\bfseries 50} \\ \hline
Entropy IDF & 0.4523&0.4270&0.4349&0.4337 \\
Normal &0.4176&0.3877&0.3724&0.3555\\
ProbIDF &0.3773&0.3712&0.3608&0.3705\\
LDA &0.4634&0.4518&0.4489&0.4481 \\
Normal-alt &0.4784&0.4493&0.4515&0.4420\\
\hline
\end{tabular}%
}
\end{table}
\lipsum[1]
\begin{table}[ht]
\centering
\newcommand{\?}{\textbf}
\begin{tabular}{|c|c|c|c|}
\hline
{\bfseries GTW} & {\bfseries 10} & {\bfseries 15} & {\bfseries 20} \\
\hline
Entropy IDF & 0.4301 & 0.4552 & 0.4627 \\
Normal & \?{0.4856} & 0.4802 & 0.4769 \\
ProbIDF & 0.3333 & 0.3628 & 0.3717 \\
LDA & \?{0.4668} & 0.4587 & 0.4582 \\
Norm-alt & \?{0.5598} & 0.5066 & 0.5077 \\
\hline
\end{tabular}
\begin{tabular}{|c|c|c|c|}
\hline
{\bfseries GTW} & {\bfseries 25} & {\bfseries 30} & {\bfseries 35} \\
\hline
Entropy IDF & \?{0.4648} & 0.4610 & 0.4523 \\
Normal & 0.4266 & 0.4181 & 0.4176 \\
ProbIDF & \?{0.3831} & 0.3697 & 0.3773 \\
LDA & 0.4482 & 0.4378 & 0.4634 \\
Norm-alt & 0.4685 & 0.4876 & 0.4784 \\
\hline
\end{tabular}
\begin{tabular}{|c|c|c|c|c|c}
\hline
{\bfseries GTW} & {\bfseries 40} & {\bfseries 45} & {\bfseries 50} \\
\hline
Entropy IDF & 0.4270 & 0.4349 & 0.4337 \\
Normal & 0.3877 & 0.3724 & 0.3555 \\
ProbIDF & 0.3712 & 0.3608 & 0.3705 \\
LDA & 0.4518 & 0.4489 & 0.4481 \\
Normal-alt & 0.4493 & 0.4515 & 0.4420 \\
\hline
\end{tabular}
\end{table}
\lipsum
\end{document}

Please note that, notwithstanding what the people at EUSFLAT think, the command \bf
has been deprecated for 25 years.
I used \?
(a command local to the table
environment to ease input but, above all, its readability.
eusflat2021
?).twocolumn
mode using\begin{table*}
if this is an option.